redesigned tempstring system, now uses a fixed size buffer based on the prvm_tempstri...

redesigned tempstring system, now uses a fixed size buffer based on the prvm_tempstringmemory cvar (this is probably safe to automatically resize in the future but is not currently resized)
audited PRVM_G_STRING use in the engine (eliminated many NULL checks in functions because it never returns NULL)
added DP_QC_UNLIMITEDTEMPSTRINGS extension to indicate this new tempstring system's wonderous features (not yet documented)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@6746 d7cf8633-e32d-0410-b094-e92efae38249