overhaul cancellation to fix resource leaks and dangerous behavior with signals

this commit addresses two issues:

1. a race condition, whereby a cancellation request occurring after a
syscall returned from kernelspace but before the subsequent
CANCELPT_END would cause cancellable resource-allocating syscalls
(like open) to leak resources.

2. signal handlers invoked while the thread was blocked at a
cancellation point behaved as if asynchronous cancellation mode wer in
effect, resulting in potentially dangerous state corruption if a
cancellation request occurs.

the glibc/nptl implementation of threads shares both of these issues.

with this commit, both are fixed. however, cancellation points
encountered in a signal handler will not be acted upon if the signal
was received while the thread was already at a cancellation point.
they will of course be acted upon after the signal handler returns, so
in real-world usage where signal handlers quickly return, it should
not be a problem. it's possible to solve this problem too by having
sigaction() wrap all signal handlers with a function that uses a
pthread_cleanup handler to catch cancellation, patch up the saved
context, and return into the cancellable function that will catch and
act upon the cancellation. however that would be a lot of complexity
for minimal if any benefit...

src/thread/pthread_cond_timedwait.c

@@ -8,19 +8,21 @@ static void relock(void *m)
 int pthread_cond_timedwait(pthread_cond_t *c, pthread_mutex_t *m, const struct timespec *ts)
 {
 	int r, e=0;
-	CANCELPT(0);
+	CANCELPT_BEGIN;
+	CANCELPT_END;
 
 	pthread_cleanup_push(relock, m);
 	c->_c_block = 1;
 	if ((r=pthread_mutex_unlock(m))) return r;
 
-	CANCELPT(1);
+	CANCELPT_BEGIN;
 	e = __timedwait(&c->_c_block, 1, c->_c_clock, ts, 0);
-	CANCELPT(0);
+	CANCELPT_END;
 
 	pthread_cleanup_pop(0);
 	if ((r=pthread_mutex_lock(m))) return r;
 
-	CANCELPT(0);
+	CANCELPT_BEGIN;
+	CANCELPT_END;
 	return e;
 }