fixed multisession crash when the new cart needed a dynamic buffer/array
larger than the original one. this got introduced when converting from 
auto_alloc_array to dynamic buffers, because such buffers are not made
NULL at reset, but got unnoticed/unreported up to now. many thanks to 
Trebor for catching it! nw.
Fixed build. (nw)
trident: added XOR drawing, added XNEG and YNEG reverse directions for mono pattern BitBLT.
(MESS) Fixed ROM name conflict. (nw)
(MESS) a7000: Renamed to riscpc.c and added missing variants. (nw)
(MESS) riscpc: Added missing regions. (nw)
documented some disabled clang compiler warnings / re-enabled -Wformat-security for clang 3.5 (was disabled because of a bug in clang) / disabled c++11 warnings so it compiles out-of-the-box / added TODO (nw)
documented more disabled clang compiler warnings (nw)
fixed clang warning in src/mame/video/jagblit.inc (nw)