Fix race conditions running external commands with job control on

When ksh is compiled with SHOPT_SPAWN (the default), which uses
posix_spawn(3) or vfork(2) (via sh_ntfork()) to launch external
commands, at least two race conditions occur when launching
external commands while job control is active. See:
1887863/comments/3
https://www.mail-archive.com/ast-developers@research.att.com/msg00717.html

The basic issue is that this performance optimisation is
incompatible with job control, because it uses a spawning mechanism
that doesn't copy the parent process' memory pages into the child
process, therefore no state that involves memory can be set before
exec-ing the external program. This makes it impossible to
correctly set the terminal's process group ID in the child process,
something that is essential for job control to work.

src/cmd/ksh93/sh/xec.c:
- Use sh_fork() instead of sh_ntfork() if job control is active.
  This uses fork(2), which is 30%-ish slower on most sytems, but
  allows for correctly setting the terminal process group.

src/cmd/ksh93/tests/basic.sh:
- Add regression test for the race condition reported in #79.

src/cmd/INIT/cc.darwin:
- Remove hardcoded flag to disable SHOPT_SPAWN on the Mac.
  It should be safe to use now.

Fixes https://github.com/ksh93/ksh/issues/79

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.
 
+2020-07-22:
+
+- Fixed two race conditions when running external commands on
+  interactive shells with job control active.
+
 2020-07-20:
 
 - If a shell function and a built-in command by the same name exist,