transition to using functions for internal signal blocking/restoring

there are several reasons for this change. one is getting rid of the
repetition of the syscall signature all over the place. another is
sharing the constant masks without costly GOT accesses in PIC.

the main motivation, however, is accurately representing whether we
want to block signals that might be handled by the application, or all
signals.

src/internal/pthread_impl.h

@@ -115,6 +115,10 @@ void __acquire_ptc();
 void __release_ptc();
 void __inhibit_ptc();
 
+void __block_all_sigs(void *);
+void __block_app_sigs(void *);
+void __restore_sigs(void *);
+
 #define DEFAULT_STACK_SIZE 81920
 #define DEFAULT_GUARD_SIZE PAGE_SIZE