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/stdio/gets.c

@@ -19,6 +19,6 @@
 #include "libc/limits.h"
 #include "libc/stdio/stdio.h"
 
-compatfn char *gets(char *s) {
+char *gets(char *s) {
   return fgets(s, INT_MAX, stdin);
 }