fs: Add file IO systemcalls

System calls are now coupled to their values, although they remain
unimplemented.

VFS changes have to be made in order to complete the system calls.

Signed-off-by: Bart Kuivenhoven <bemkuivenhoven@gmail.com>
lib: tree: Fix function names and return types

Signed-off-by: Bart Kuivenhoven <bemkuivenhoven@gmail.com>