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

src/cmd/ksh93/sh/xec.c

@@ -1575,7 +1575,7 @@ int sh_exec(register const Shnode_t *t, int flags)
 				}
 #if SHOPT_SPAWN
 #   ifdef _lib_fork
-				if(com)
+				if(com && !job.jobcontrol)
 					parent = sh_ntfork(shp,t,com,&jobid,ntflag);
 				else
 					parent = sh_fork(shp,type,&jobid);
@@ -3460,6 +3460,7 @@ static void sigreset(Shell_t *shp,int mode)
 
 /*
  * A combined fork/exec for systems with slow or non-existent fork()
+ * Note: Incompatible with job control.
  */
 static pid_t sh_ntfork(Shell_t *shp,const Shnode_t *t,char *argv[],int *jobid,int flag)
 {