Add a new variable .SHELLSTATUS which holds the exit status of the
last-invoked shell function or != assignment.

* NEWS, doc/make.texi: Document the change.
* function.c (shell_completed, msdos_openpipe, func_shell_base): Add
shell_completed() to handle the completion of the shell, by setting
.SHELLSTATUS.  Call it where needed.
* job.c (child_handler): Call shell_completed().
* tests/scripts/functions/shell: Add tests for .SHELLSTATUS.