Header file clean-up:

Header file clean-up:
1. Remove all.h and PC_HEADER junk.
2. Rolled mem.c and mem.h into imports.c and imports.h
3. Include imports.h instead of mem.h
Restore _mesa_create/initialize_context() to be like they were in 4.0.4
New wrappers for a few std C functions: _mesa_atoi(), _mesa_strstr(), etc.