LibC: Rewrite all the headers.

We now have more or less posix issue 2018 conforming libc headers.

This was a really time consuming and boring operation but it had to
be done.

Now we get to actually start implementing libc :)

libc/unistd.cpp

@@ -1,4 +1,5 @@
 #include <BAN/Assert.h>
+#include <kernel/Syscall.h>
 #include <errno.h>
 #include <stdarg.h>
 #include <stddef.h>