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/tests/basic.sh

@@ -598,5 +598,22 @@ eu=$(
 )
 [[ ${us:1:1} == ${eu:1:1} ]] && err_exit "The time keyword ignores the locale's radix point (both are ${eu:1:1})"
 
+# ======
+# Test for bug in ksh binaries that use posix_spawn() while job control is active.
+# See discussion at: https://github.com/ksh93/ksh/issues/79
+if	test -t 1 2>/dev/null 1>/dev/tty	# this test only works if we have a tty
+then	actual=$(
+		"$SHELL" -i <<-\EOF 2>/dev/tty
+		printf '%s\n' 1 2 3 4 5 | while read
+		do	ls /dev/null
+		done 2>&1
+		exit  # suppress extra newline
+		EOF
+	)
+	expect=$'/dev/null\n/dev/null\n/dev/null\n/dev/null\n/dev/null'
+	[[ $actual == "$expect" ]] || err_exit 'Race condition while launching external commands' \
+		"(expected $(printf %q "$expect"), got $(printf %q "$actual"))"
+fi
+
 # ======
 exit $((Errors<125?Errors:125))