Add more POSIX I/O functions

[mbitsnbites]

There are a few useful POSIX I/O functions that can complement the functionality of the MFAT library.

Deleting files:

• unlink

Creating/deleting directories:

• mkdir
• rmdir

Directory traversal:

• opendir
• readdir
• closedir

Also consider fd-based vs path-based functions (fdopendir, openat, ...).