Move file initialization from files.c to empfile oninit()

Move file initialization from files.c to empfile oninit()

files.c writes initial contents to game state files with fixed size.
Necessary for setting up invariants, such as struct sctstr members
sct_x, sct_y matching sct_uid.

Do that from the oninit() callback, so ef_blank() sets up invariants
correctly.  Since ef_blank() isn't used for these files right now,
this isn't a bug fix, just cleanup.