rewrote most of the DP_QC_STRINGBUFFERS implementation

rewrote most of the DP_QC_STRINGBUFFERS implementation
string buffers are now associated with their parent progs (and no longer
leak when the progs is freed)
removed almost all limits on string buffers (there is a sanity limit of
a million strings per buffer)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7971 d7cf8633-e32d-0410-b094-e92efae38249