Fix Windows-specific use-after-free causing crash after disconnecting

Fix Windows-specific use-after-free causing crash after disconnecting

This explicitly deregisters CSQC commands every time CSQC shuts down. This patch
also avoids Z_Malloc'ing the cmd names for each CSQC command and avoids forcing
a Cmd_AddCommand if the command exists.

CSQC was force-adding a command even if it already existed, and when it did,
it would pass a pointer from a mempool that would later get freed when
CSQC shuts down, and Windows' strcasecmp doesn't like dangling pointers.
The Z_Malloc wasn't much better because it could have caused a memory leak.
So the best solution was to only pass the pointer but make sure the commands
are freed when CSQC shuts down.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12558 d7cf8633-e32d-0410-b094-e92efae38249