Kernel+LibC: Share definitions in fcntl.h and sys/types.h

This patch begins the work of sharing types and macros between Kernel
and LibC instead of duplicating them via the kludge in UnixTypes.h.

The basic idea is that the Kernel vends various POSIX headers via
Kernel/API/POSIX/ and LibC simply #include's them to get the macros.

Userland/Libraries/LibC/fcntl.cpp

@@ -7,6 +7,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <stdarg.h>
 #include <string.h>
 #include <syscall.h>