libroot: Implement a "partial" vfork.

The original meaning of vfork is "fork, sharing virtual memory" (until
exec). We don't implement that, and may never do so. However, since
calling any functions besides exec() in a vfork'ed child is "undefined
behavior", we can take advantage of that fact at least by not calling
any of the pre- and post-fork hooks, saving a lot of page faults from
copy-on-write.

On one run of the "compile HaikuDepot and the mime_db" benchmark with -j4,
the total waits count on the top two VMCaches by contention dropped
from 62125 and 58927, to 52034 and 41225.

musl apparently does more or less this same thing (vfork() is fork()
but without calling any of the hooks.)

src/system/libroot/posix/unistd/fork.c

@@ -185,6 +185,11 @@ fork(void)
 pid_t
 vfork(void)
 {
-	return fork();
+	thread_id thread = _kern_fork();
+	if (thread < 0) {
+		// something went wrong
+		__set_errno(thread);
+		thread = -1;
+	}
+	return thread;
 }
-