s/vm_page_size/vm_get_page_size/ to avoid name clash on MacOS X

Enable 33-bit memory addressing on 64-bit JIT capable platforms (e.g. x86-64).
This is useful to get rid of address offset sign extensions. It uses POSIX
shared memory to create aliased regions, fallback to usual sign-extension
way if shm_open et al. don't work (e.g. no /dev/shm mounted)

Enable possibility to manually pass VM_MAP_32BIT & friends. This is needed
for the PowerPC "JIT1" engine.

- Also include <sys/types.h> if system has <unistd.h>

- added SIGSEGV support for Linux/Alpha (to be checked), Darwin/PPC
- added uniform virtual memory allocation
  (supports mmap(), vm_allocate(), or fallbacks to malloc()/free())
- cleaned up memory allocation in main_unix.cpp