new framework to inhibit thread cancellation when needed

with these small changes, libc functions which need to call functions
which are cancellation points, but which themselves must not be
cancellation points, can use the CANCELPT_INHIBIT and CANCELPT_RESUME
macros to temporarily inhibit all cancellation.

src/internal/libc.h

@@ -45,6 +45,8 @@ void __lockfile(FILE *);
 #define CANCELPT_BEGIN CANCELPT(1)
 #define CANCELPT_TRY CANCELPT(0)
 #define CANCELPT_END CANCELPT(-1)
+#define CANCELPT_INHIBIT CANCELPT(2)
+#define CANCELPT_RESUME CANCELPT(-2)
 
 extern char **__environ;
 #define environ __environ