games/doom: implement filelength() (thanks quux)

this function is used when playing demos from external lumps. the game just
exits without this patch.
to test this, download a demo lump from somewhere, and play it with -playdemo %s
where %s is the file's name, without the .lmp extension:

(note that this one is a doom 2 demo, so it requires doom2.wad)
% hget http://doomedsda.us/lmps/945/3/30nm2939.zip | unzip -sv
extracting 30nm2939.LMP
extracting 30nm2939.txt
% mv 30nm2939.LMP 30nm2939.lmp	# checking for a lump filename is case sensitive
% games/doom -playdemo 30nm2939

the game exits when the demo ends. also, note that this demo will desync on
map06 (the crusher), because of an unrelated bug (that's another patch :>)

note: filelength() returns vlong, but file lengths for doom lumps are ints.
however, this might be used elsewhere (networking), so i'd leave it this way.

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