Fix crash/freeze upon interrupting command substitution with pipe

On some systems (at least Linux and macOS):

1. Run on a command line: t=$(sleep 10|while :; do :; done)
2. Press Ctrl+C in the first 10 seconds.
3. Execute any other command substitution. The shell crashes.

Analysis: Something in the job_wait() call in the sh_subshell()
restore routine may be interrupted by a signal such as SIGINT on
Linux and macOS. Exactly what that interruptible thing is remains
to be determined. In any case, since job_wait() was invoked after
sh_popcontext(), interrupting it caused the sh_subshell() restore
routine to be aborted, resulting in an inconsistent state of the
shell. The fix is to sh_popcontext() at a later stage instead.

src/cmd/ksh93/sh/subshell.c: sh_subshell():
- Rename struct checkpt buff to checkpoint because it's clearer.
- Move the sh_popcontext() call to near the end, just after
  decreasing the subshell level counters and restoring the global
  subshell data struct to its parent. This seems like a logical
  place for it and could allow other things to be interrupted, too.
- Get rid of the if(shp->subshell) because it is known that the
  value is > 0 at this point.
- The short exit routine run if the subshell forked now needs a new
  sh_popcontext() call, because this is handled before restoring
  the virtual subshell state.
- While we're here, do a little more detransitioning from all those
  pointless shp pointers.

Fixes: https://github.com/ksh93/ksh/issues/397

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-12-27:
+
+- Fixed a crash or freeze that would occur on Linux when using Ctrl+C to
+  interrupt a command substitution containing a pipe in an interactive shell.
+
 2021-12-26:
 
 - Listing aliases or tracked aliases in a script no longer corrupts