setup: Make iwad_t pointers const.

The table of IWAD data is stored in const memory, so make all iwad_t
pointers const to fix compiler warnings.
server: Fix sending of WAD/DEH checksums to clients.

Clients receive the WAD and dehacked checksums of the controlling
player and are supposed to display a warning at the startup screen if
they do not match. However, reversed logic in the code that sends the
waiting data to clients meant that they were always sent their own
checksums, so the error message was never displayed.

This fixes #384.