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.

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.
 
+2020-09-23:
+
+- Fixed a crash that could occur when running a pipeline containing
+  backtick-style command substitutions with job control enabled.
+
 2020-09-21:
 
 - A bug was fixed that caused command substitutions embedded in here-documents