Merge pull request #425 from ImperatorPrime/master

Migrated JK2 stuff to JKA equivalents.
[SP] Removed isJK2Engine from COM_BeginParseSession.  Fixed uses in jk2 code.
[SP] Fix button binds for JK2 mode.
[MP] Merged a fix from ioq3 that hopefully fixes reconnect exploit with duels.
Fix comes from ioquake/ioq3@82f79897 (Not full commit)
[MP] Map vote: Make sure to close file if it was empty but, a valid handle.
Removed unused _USRDLL define and dependent code.