pty: Fix signal handling (re: 1ca9286a)

This applies the OpenSUSE changes to pty.c from:
https://build.opensuse.org/package/view_file/shells/ksh/ksh93-builtin.dif

src/cmd/builtin/pty.c:
- Add a patch from OpenSUSE with a fix for signal handling.

src/cmd/ksh93/tests/pty.sh:
- Re-enable the process group exercise test.

Resolves: https://github.com/ksh93/ksh/issues/61

src/cmd/ksh93/tests/pty.sh

@@ -463,12 +463,7 @@ r echo repeat-3
 !
 fi
 
-# Following test is disabled because a bug in pty causes it to fail on too
-# many operating systems. Apparently pty doesn't handle SIGTSTP correctly:
-# https://github.com/att/ast/issues/375
-# TODO: fix pty and re-enable this test.
-: <<\end_disabled
-# err_(don't count me)_exit #
+# err_exit #
 whence -q less &&
 TERM=vt100 tst $LINENO <<"!"
 L process/terminal group exercise
@@ -480,7 +475,6 @@ r Stopped
 w fg
 u yes-yes
 !
-end_disabled
 
 # err_exit #
 # Test file name completion in vi mode