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/internal/libc.h

@@ -44,7 +44,8 @@ void __lockfile(FILE *);
 #define UNLOCK(x) (*(x)=0)
 #define CANCELPT(x) (libc.cancelpt ? libc.cancelpt((x)),0 : (void)(x),0)
 #define CANCELPT_BEGIN CANCELPT(1)
-#define CANCELPT_END CANCELPT(0)
+#define CANCELPT_TRY CANCELPT(0)
+#define CANCELPT_END CANCELPT(-1)
 
 extern char **__environ;
 #define environ __environ