pthread and synccall cleanup, new __synccall_wait op

fix up clone signature to match the actual behavior. the new
__syncall_wait function allows a __synccall callback to wait for other
threads to continue without returning, so that it can resume action
after the caller finishes. this interface could be made significantly
more general/powerful with minimal effort, but i'll wait to do that
until it's actually useful for something.

src/internal/libc.h

@@ -41,6 +41,7 @@ void __unlockfile(FILE *);
 #define UNLOCK(x) (*(volatile int *)(x)=0)
 
 void __synccall(void (*)(void *), void *);
+void __synccall_wait(void);
 int __setxid(int, int, int, int);
 
 extern char **__environ;