Fix side effect to exit status of DEBUG trap in comsub

This fixes the following:

trap ':' DEBUG
r=$(exit 123)
echo $? # Expected 123, but actually 0.

Thanks to Koichi Nakashima for the report and reproducer.

src/cmd/ksh93/sh/fault.c: sh_trap():
- Restore the saved current exit status (exitval) for all traps.
  Do not except the DEBUG trap from doing that. I've no idea why
  this exception was made, but it's not correct.

src/cmd/ksh93/tests/basic.sh:
- Add tests.

Makes progress on: https://github.com/ksh93/ksh/issues/155

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.
 
+2021-01-20:
+
+- Fixed: executing a DEBUG trap in a command substitution had side effects
+  on the exit status ($?) of non-trap commands.
+
 2021-01-19:
 
 - Fixed a crash when using 'cd' in a virtual/non-forking subshell in a