Make important improvements

- Fix preadv() and pwritev() for old distros
- Introduce _npassert() and _unassert() macros
- Prove that file locks work properly on Windows
- Support fcntl(F_DUPFD_CLOEXEC) on more systems

libc/intrin/describemagnums.c

@@ -22,6 +22,7 @@
 
 char *DescribeMagnum(char *b, const struct MagnumStr *m, const char *p, int x) {
   char *s;
+  if (x == 127) return "CLOCK_INVALID";
   if ((s = GetMagnumStr(m, x))) {
     stpcpy(stpcpy(b, p), s);
     return b;