server: always use correct case for map names

server: always use correct case for map names

Previously whatever was typed in as the map/changelevel argument was
used even if it didn't match the case of the filename in the pak, which
meant QC could see multiple instances of the same map that differed only
by case, causing problems for gameplay stats.

Now DP will always use the same case as the real .bsp filename for the
`mapname` globals, and the sv_world* and cl_world* cvars, and the server
status and getinfo/getstatus responses.  This matches the behaviour of
QC code which retrieved the name from the VFS, such as the `gotomap` and
voting code in Nexuiz and Xonotic.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>