Kernel+LibC: Implement the openat() syscall

POSIX's openat() is very similar to open(), except you also provide a
file descriptor referring to a directory from which relative paths
should be resolved.

Passing it the magical fd number AT_FDCWD means "resolve from current
directory" (which is indeed also what open() normally does.)

This fixes libarchive's bsdtar, since it was trying to do something
extremely wrong in the absence of openat() support. The issue has
recently been fixed upstream in libarchive:

https://github.com/libarchive/libarchive/issues/1239

However, we should have openat() support anyway, so I went ahead and
implemented it. :^)

Fixes #748.

Kernel/Process.h

@@ -124,6 +124,7 @@ public:
     pid_t sys$getppid();
     mode_t sys$umask(mode_t);
     int sys$open(const Syscall::SC_open_params*);
+    int sys$openat(const Syscall::SC_openat_params*);
     int sys$close(int fd);
     ssize_t sys$read(int fd, u8*, ssize_t);
     ssize_t sys$write(int fd, const u8*, ssize_t);