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/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;