file_sys: Add factory class for RomFS file system.

and make the mode parameter a reference since it is a BitField union

This makes clang-format useful on those.

Also add a bunch of forgotten transitive includes, which otherwise
prevented compilation.

These operations are limited by the open flags specified while opening the file.

Buffer lengths should be size_t, and file offsets should be u64.

Fixed a few warnings and cleaned up the code