Fix crash in backtick comsubs with job control on (rhbz#825520)

This imports another fix from Red Hat/Fedora. Original patch:
642af4d6/f/ksh-20120801-crash.patch

src/cmd/ksh93/include/jobs.h,
src/cmd/ksh93/sh/jobs.c,
src/cmd/ksh93/sh/subshell.c,
src/cmd/ksh93/sh/xec.c:
- Import the Red Hat fix with these differences:
  - Rename the 'hack1_waitall' variable to 'bktick_waitall' and add
    a comment describing what it's for.
  - Remove unused 'pipefail' variable.

src/cmd/ksh93/tests/basic.sh:
- Regression test from reproducer given in the Red Hat bug report.
- Add special handling to SIGKILL it, as it might freeze hard.

src/cmd/ksh93/include/jobs.h

@@ -99,6 +99,7 @@ struct jobs
 	char		jobcontrol;	/* turned on for real job control */
 	char		waitsafe;	/* wait will not block */
 	char		waitall;	/* wait for all jobs in pipe */
+	char		bktick_waitall;	/* wait state for `backtick comsubs` */
 	char		toclear;	/* job table needs clearing */
 	unsigned char	*freejobs;	/* free jobs numbers */
 };

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-09-21"
+#define SH_RELEASE	"93u+m 2020-09-23"