clone(2): fix check for attempts to create new process

The existing clone(2) syscall implementation was detecting whether
the user program is trying to create a new process by checking the
child_stack argument against a null value. This logic is based on
the fact that the glibc fork() wrapper that is provided as part of
the NPTL threading implementation invokes clone(2) with child_stack
set to 0. However, it is possible to create a new process even if
child_stack is non-zero: notably, the posix_spawn() implementation
in glibc invokes clone() with a valid stack pointer, which is
later unmapped by the parent process after the child starts
executing the newly created process. The correct way for clone(2)
to detect an attempt to create a new process is by checking for the
CLONE_THREAD flag.

src/unix/thread.c

@@ -60,8 +60,8 @@ sysreturn clone(unsigned long flags, void *child_stack, int *ptid, unsigned long
     thread_log(current, "clone: flags %lx, child_stack %p, ptid %p, ctid %p, newtls %lx",
         flags, child_stack, ptid, ctid, newtls);
 
-    if (!child_stack) {   /* this is actually a fork() */
-        thread_log(current, "attempted to fork by passing null child stack, aborting.");
+    if (!(flags & CLONE_THREAD)) {
+        thread_log(current, "attempted to create new process, aborting.");
         return set_syscall_error(current, ENOSYS);
     }