fix minor bugs due to incorrect threaded-predicate semantics

some functions that should have been testing whether pthread_self()
had been called and initialized the thread pointer were instead
testing whether pthread_create() had been called and actually made the
program "threaded". while it's unlikely any mismatch would occur in
real-world problems, this could have introduced subtle bugs. now, we
store the address of the main thread's thread descriptor in the libc
structure and use its presence as a flag that the thread register is
initialized. note that after fork, the calling thread (not necessarily
the original main thread) is the new main thread.

src/internal/libc.h

@@ -16,6 +16,7 @@ struct __libc {
 	volatile int threads_minus_1;
 	int ofl_lock;
 	FILE *ofl_head;
+	void *main_thread;
 };
 
 

src/process/fork.c

@@ -8,10 +8,11 @@ pid_t fork(void)
 	pid_t ret;
 	if (libc.fork_handler) libc.fork_handler(-1);
 	ret = syscall(SYS_fork);
-	if (libc.threaded && !ret) {
+	if (libc.main_thread && !ret) {
 		pthread_t self = __pthread_self();
 		self->tid = self->pid = syscall(SYS_getpid);
 		libc.threads_minus_1 = 0;
+		libc.main_thread = self;
 	}
 	if (libc.fork_handler) libc.fork_handler(!ret);
 	return ret;

src/thread/pthread_self.c

@@ -6,7 +6,6 @@ static struct pthread main_thread;
 static const void *dummy[1] = { 0 };
 weak_alias(dummy, __pthread_tsd_main);
 
-#undef errno
 static int *errno_location()
 {
 	return __pthread_self()->errno_ptr;
@@ -14,11 +13,10 @@ static int *errno_location()
 
 static int init_main_thread()
 {
+	if (__set_thread_area(&main_thread) < 0) return -1;
 	main_thread.canceldisable = libc.canceldisable;
 	main_thread.tsd = (void **)__pthread_tsd_main;
-	main_thread.self = &main_thread;
-	if (__set_thread_area(&main_thread) < 0)
-		return -1;
+	main_thread.self = libc.main_thread = &main_thread;
 	main_thread.errno_ptr = __errno_location();
 	libc.errno_location = errno_location;
 	main_thread.tid = main_thread.pid = 

src/thread/pthread_setcancelstate.c

@@ -3,7 +3,7 @@
 int pthread_setcancelstate(int new, int *old)
 {
 	if (new > 1U) return EINVAL;
-	if (libc.threaded) {
+	if (libc.main_thread) {
 		struct pthread *self = __pthread_self();
 		if (old) *old = self->canceldisable;
 		self->canceldisable = new;