Eliminate libc/dirent directory

All of our dirent functions (readdir, opendir, etc.) are our own
implementation, in fs/vfs/main.cc, and not taken from Musl.
Therefore it is silly we only took one small function, fdopendir.c,
from musl, and even there we had to modify that function to fit our code.

So this patch eliminates the libc/dirent directory - fdopendir.c and
dirent.h, and instead puts the tiny fdopendir() implementation with
all its friends, in fs/vfs/main.cc.

This patch also takes include/api/dirent.h unmodified from musl.
We previously had small modifications to that file, and there is
no real justification for that.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>