Fix ksh exit on syntax error in profile (re: cb67a01b, ceb77b13)

Johnothan King writes:
> There are two regressions related to how ksh handles syntax
> errors in the .kshrc file. If ~/.kshrc or the file pointed to by
> $ENV have a syntax error, ksh exits during startup. Additionally,
> the error message printed is incorrect:
>
> $ cat /tmp/synerror
> ((
> echo foo
>
> # ksh93u+m
> $ ENV=/tmp/synerror arch/*/bin/ksh -ic 'echo ${.sh.version}'
> /tmp/synerror: syntax error: `/t/tmp/synerror' unmatched
>
> # ksh93u+
> $ ENV=/tmp/synerror ksh93u -ic 'echo ${.sh.version}'
> /tmp/synerror: syntax error: `(' unmatched
> Version AJM 93u+ 2012-08-01
>
> The regression that causes the incorrect error message was
> introduced by commit cb67a01. The other bug that causes ksh to
> exit on startup was introduced by commit ceb77b1.

src/cmd/ksh93/sh/lex.c: fmttoken():
- Call stakfreeze(0) to terminate a possible unterminated previous
  stack item before writing the token string onto the stack. This
  fixes the bug with garbage in a syntax error message.

src/cmd/ksh93/sh/main.c: exfile():
- Revert Red Hat's ksh-20140801-diskfull.patch applied in ceb77b13.
  This fixes the bug with interactive ksh exiting on syntax error
  in a profile script. Testing by @JohnoKing showed the patch is no
  longer necessary to fix a login crash on disk full, as commit
  970069a6 (which applied Red Hat patches ksh-20120801-macro.patch
  and ksh-20120801-fd2lost.patch) also fixes that crash.

src/cmd/ksh93/README:
- Fix typos. (re: fdc08b23)

Co-authored-by: Johnothan King <johnothanking@protonmail.com>
Resolves: https://github.com/ksh93/ksh/issues/281

src/cmd/ksh93/tests/pty.sh

@@ -797,5 +797,19 @@ w cd emacste\t123abc
 r ^:test-1: cd emacstest/123abc\r\n$
 !
 
+# err_exit #
+echo '((' >$tmp/synerror
+ENV=$tmp/synerror tst $LINENO <<"!"
+L syntax error in profile causes exit on startup
+# https://github.com/ksh93/ksh/issues/281
+
+d 15
+r /synerror: syntax error: `\(' unmatched\r\n$
+p :test-1:
+w echo ok
+r ^:test-1: echo ok\r\n$
+r ^ok\r\n$
+!
+
 # ======
 exit $((Errors<125?Errors:125))