Big(gish) map reorganization.

Moved all main .dmm maps to _maps/map_files
Each main map gets a .dm file in _maps, this is what's ticked, -NOT- the .dmm file.
This allows use of a MAP_NAME define for various purposes, and lays the groundwork for map customization in the future.

Also included a hacky dm.sh script which supports swapping maps.
For example, "./dm.sh tgstation.dme -Mmetastation" would compile with metastation.

Signed-off-by: Mloc-Argent <colmohici@gmail.com>