Fix KEYBD trap crash when inputting a command substitution (#355)

This change fixes a crash that can occur after setting a KEYBD trap
then inputting a multi-line command substitution. The crash is
similar to issue #347, but it's easier to reproduce since it
doesn't require you to setup a kshrc file. Reproducer for the
crash:

  $ ENV=/./dev/null ksh
  $ trap : KEYBD
  $ : $(
  > true)
  Memory fault(coredump)

The bugfix was backported (with considerable changes) from ksh93v-
2013-10-08. The crash was first reported on the old mailing list:
https://www.mail-archive.com/ast-users@lists.research.att.com/msg00313.html

src/cmd/ksh93/{include/shlex.h,sh/lex.c}:
- To fix this properly, we need sizeof(Lex_t) to work as expected
  in edit.c, but that is thwarted by the _SHLEX_PRIVATE macro in
  lex.c which shlex.h uses to add private structs to the Lex_t type
  in lex.c only. So get rid of that _SHLEX_PRIVATE macro and make
  those members part of the centrally defined struct, renaming them
  to make it clear they're considered private to lex.c.

src/cmd/ksh93/edit/edit.c:
- Now that we can get its size, save and restore the shell lexing
  context when a KEYBD trap is present.

src/cmd/ksh93/tests/pty.sh:
- Add a regression test for the KEYBD trap crash.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

src/cmd/ksh93/tests/pty.sh

@@ -878,5 +878,16 @@ w : test_string\1\E6\E[C\4
 r ^:test-4: : teststring\r\n$
 !
 
+# err_exit #
+tst $LINENO <<"!"
+L crash with KEYBD trap after entering multi-line command substitution
+# https://www.mail-archive.com/ast-users@lists.research.att.com/msg00313.html
+
+w trap : KEYBD
+w : $(
+w true); echo "Exit status is $?"
+u Exit status is 0
+!
+
 # ======
 exit $((Errors<125?Errors:125))