levels: new "map15a" from Z0k

Slot to be determined; author intended it to replace his own map15,
but it seems a shame to lose the existing map15 when there are much
weaker maps it could replace instead.

DW: http://www.doomworld.com/vb/post/1256159
DW: http://www.doomworld.com/vb/post/1265585
Signed-off-by: RjY <rjy@users.sourceforge.net>
levels: Z0k's map15a is map12, move latter to map24

See http://www.doomworld.com/vb/post/1303910