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/sh/jobs.c

@@ -1803,6 +1803,7 @@ again:
 	{
 		count = bp->count;
 		jp = bp->list;
+		jpold = 0;
 		goto again;
 	}
 	if(jp)

src/cmd/ksh93/sh/subshell.c

@@ -524,7 +524,10 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 	sp->comsub = shp->comsub;
 	shp->subshare = comsub==2 ||  (comsub==1 && sh_isoption(SH_SUBSHARE));
 	if(comsub)
+	{
 		shp->comsub = comsub;
+		job.bktick_waitall = (comsub==1);
+	}
 	if(!comsub || !shp->subshare)
 	{
 		struct subshell *xp;
@@ -655,6 +658,7 @@ Sfio_t *sh_subshell(Shell_t *shp,Shnode_t *t, volatile int flags, int comsub)
 		}
 		else
 		{
+			job.bktick_waitall = 0;
 			if(comsub!=1 && shp->spid)
 			{
 				job_wait(shp->spid);

src/cmd/ksh93/sh/xec.c

@@ -1927,7 +1927,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 				memset(exitval,0,job.waitall*sizeof(int));
 			}
 			else
-				job.waitall |= !pipejob && sh_isstate(SH_MONITOR);
+				job.waitall |= (job.bktick_waitall || !pipejob && sh_isstate(SH_MONITOR));
 			job_lock();
 			nlock++;
 			do