Developer build: decreased limits, new fopen() modes, DP_SMALLMEMORY documented

Developer build have several options to assist mod developers: increased
limits, no home directory (equivalent to -nohome switch),  new fopen()
for real files (pak/pk3 files are completely ignored, allowing to read
any file, including system ones, allowing to write and delete files in
game dir).
Also documented DP_BUILD_SMALLMEMORY and DP_QC_STRINGBUFFERS_EXT_WIP