Restore AST 'pty' pseudoterminal command (re: reboot)

This enables the tests/pty.sh regression tests for the interactive
shell to work. I've managed to restore these without restoring
nmake (which would cause unresolved build failures). It involved
hand-editing the Mamfile originally generated by nmake.

src/cmd/builtin/Makefile:
- Restored. Even though nmake is not installed, apparently mamake
  still needs the Makefile for the 'install' target to work. This
  will probably need editing if nmake is ever restored.

src/cmd/builtin/Mamfile:
- Restored and edited by hand to remove the commands to build all
  the utilities except pty, as well as all test targets (pty has no
  test). Most of this was systematically deleting blocks delimited
  by 'make FOO' and 'done FOO generated' until the build stopped
  failing -- a task made so much easier by the build now failing
  gracefully on error (see d0dfb37c). To see the diff, do:
	git diff cc1f2bf8 src/cmd/builtin/Mamfile

src/cmd/builtin/RELEASE:
- Restored for completeness' sake with all non-pty entries removed.

src/cmd/builtin/features/pty,
src/cmd/builtin/pty.c:
- Restored.

src/cmd/builtin/Mamfile

@@ -0,0 +1,95 @@
+info mam static 00000 1994-07-17 make (AT&T Research) 5.7 2012-06-20
+setv INSTALLROOT ../../..
+setv PACKAGE_ast_INCLUDE ${INSTALLROOT}/include/ast
+setv PACKAGE_ast_LIB ${INSTALLROOT}/lib
+setv PACKAGE_cmd ${INSTALLROOT}
+setv PACKAGE_cmd_INCLUDE ${PACKAGE_cmd}/include
+setv PACKAGE_cmd_LIB ${PACKAGE_cmd}/lib
+setv PACKAGEROOT ../../../../..
+setv AR ${mam_cc_AR} ${mam_cc_AR_ARFLAGS}
+setv ARFLAGS rc
+setv AS as
+setv ASFLAGS
+setv CC cc
+setv mam_cc_FLAGS
+setv CCFLAGS ${-debug-symbols?1?${mam_cc_DEBUG} -D_BLD_DEBUG?${mam_cc_OPTIMIZE}?}
+setv CCLDFLAGS ${-strip-symbols?1?${mam_cc_LD_STRIP}??}
+setv COTEMP $$
+setv CPIO cpio
+setv CPIOFLAGS
+setv CPP "${CC} -E"
+setv F77 f77
+setv HOSTCC ${CC}
+setv IGNORE
+setv LD ld
+setv LDFLAGS
+setv LEX lex
+setv LEXFLAGS
+setv LPR lpr
+setv LPRFLAGS
+setv M4FLAGS
+setv NMAKE nmake
+setv NMAKEFLAGS
+setv PR pr
+setv PRFLAGS
+setv SHELL /bin/sh
+setv SILENT
+setv TAR tar
+setv YACC yacc
+setv YACCFLAGS -d
+make ${PACKAGEROOT}/lib/package/ast.lic
+done ${PACKAGEROOT}/lib/package/ast.lic
+make .INIT
+make ${PACKAGE_ast_INCLUDE}/cmdlist.h
+make ${PACKAGE_ast_INCLUDE}/prototyped.h implicit
+done ${PACKAGE_ast_INCLUDE}/prototyped.h dontcare
+done ${PACKAGE_ast_INCLUDE}/cmdlist.h
+exec - sed -e '/^CMDLIST(.*)$/!d' -e 's/CMDLIST(\(.*\))/\1/' -e '/^getconf$/d' -e '/^ln$/d' -e '/^mv$/d' -e '/^md5sum$/d' -e '/^sum$/d' ${PACKAGE_ast_INCLUDE}/cmdlist.h
+bind -lcmd
+done .INIT dontcare virtual
+make install
+make pty
+make pty.o
+make pty.c
+make FEATURE/pty implicit
+meta FEATURE/pty features/%>FEATURE/% features/pty pty
+make features/pty
+done features/pty
+exec - iffe -v -c '${CC} ${mam_cc_FLAGS} ${CCFLAGS}   ${LDFLAGS} ' ref ${mam_cc_L+-L${INSTALLROOT}/lib} -I${PACKAGE_ast_INCLUDE} -I${INSTALLROOT}/include ${mam_libast} ${mam_libcmd} : run features/pty
+done FEATURE/pty generated
+make ${PACKAGE_ast_INCLUDE}/ast_time.h implicit
+done ${PACKAGE_ast_INCLUDE}/ast_time.h
+prev ${PACKAGE_ast_INCLUDE}/vmalloc.h implicit
+prev ${PACKAGE_ast_INCLUDE}/regex.h implicit
+make ${PACKAGE_ast_INCLUDE}/proc.h implicit
+prev ${PACKAGE_ast_INCLUDE}/ast.h implicit
+prev ${PACKAGE_ast_INCLUDE}/prototyped.h implicit
+done ${PACKAGE_ast_INCLUDE}/proc.h
+prev ${PACKAGE_ast_INCLUDE}/error.h implicit
+prev ${PACKAGE_ast_INCLUDE}/cmd.h implicit
+done pty.c
+meta pty.o %.c>%.o pty.c pty
+prev pty.c
+setv CMD_STANDALONE -DCMD_STANDALONE="b_pty"
+setv LICENSE -DLICENSE="since=2001,author=gsf+dgk"
+exec - ${CC} ${mam_cc_FLAGS} ${CCFLAGS} -I. -I${PACKAGE_ast_INCLUDE} -DERROR_CATALOG=\""builtin"\" -D_PACKAGE_ast -DCMD_STANDALONE=b_pty -DUSAGE_LICENSE=\""[-author?Glenn Fowler <gsf@research.att.com>][-author?David Korn <dgk@research.att.com>][-copyright?Copyright (c) 2001-2012 AT&T Intellectual Property][-license?http://www.eclipse.org/org/documents/epl-v10.html][--catalog?builtin]"\" -c pty.c
+done pty.o generated
+bind -lutil dontcare
+setv CMD_STANDALONE -DCMD_STANDALONE="b_pty"
+exec - ${CC} ${CCLDFLAGS} ${mam_cc_FLAGS} ${CCFLAGS} ${LDFLAGS} ${mam_cc_L+-L.} ${mam_cc_L+-L${INSTALLROOT}/lib} -o pty pty.o ${mam_libutil} ${mam_libast} ${mam_libcmd}
+done pty generated
+make ${INSTALLROOT}/bin
+exec - if silent test ! -d ${INSTALLROOT}/bin
+exec - then mkdir -p ${INSTALLROOT}/bin
+exec - fi
+done ${INSTALLROOT}/bin generated
+make ${INSTALLROOT}/bin/pty
+prev pty
+exec - test '' = 'pty' || ${STDCMP} 2>/dev/null -s pty ${INSTALLROOT}/bin/pty || { ${STDMV} ${INSTALLROOT}/bin/pty ${INSTALLROOT}/bin/pty.old 2>/dev/null || true; ${STDCP} pty ${INSTALLROOT}/bin/pty ;}
+done ${INSTALLROOT}/bin/pty generated
+make ${INSTALLROOT}/bin
+exec - if silent test ! -d ${INSTALLROOT}/bin
+exec - then mkdir -p ${INSTALLROOT}/bin
+exec - fi
+done ${INSTALLROOT}/bin virtual
+done install virtual