fs: Filesystem module

This PR creates a common interface for all filesystems to use.

STILL WIP
Well the idea is to only compare up to the length of the mount point's path. As far as I understand it `strcmp()` would yield `< 0` if I compare for example `fs_table[i].mount_point == "/mnt"` with `path == "/mnt/where/ever/files/go"`. But about `!= 0` you are right