Kernel+LibC: Implement futimens(3)

Implement futimes() in terms of utimensat(). Now, utimensat() strays
from POSIX compliance because it also accepts a combination of a file
descriptor of a regular file and an empty path. utimensat() then uses
this file descriptor instead of the path to update the last access
and/or modification time of a file. That being said, its prior behavior
remains intact.

With the new behavior of utimensat(), `path` must point to a valid
string; given a null pointer instead of an empty string, utimensat()
sets `errno` to `EFAULT` and returns a failure.

Userland/Libraries/LibC/stat.cpp

@@ -109,4 +109,10 @@ int fstatat(int fd, char const* path, struct stat* statbuf, int flags)
 {
     return do_stat(fd, path, statbuf, !(flags & AT_SYMLINK_NOFOLLOW));
 }
+
+// https://pubs.opengroup.org/onlinepubs/9699919799/functions/futimens.html
+int futimens(int fd, struct timespec const times[2])
+{
+    return utimensat(fd, "", times, 0);
+}
 }