This patch fixes the posix module for large file support mainly on Win64,
although some general cleanup is done as well.

The changes are:
- abstract stat->STAT, fstat->FSTAT, and struct stat->STRUCT_STAT
  This is because stat() etc. are not the correct functions to use on Win64
  (nor maybe on other platforms?, if not then it is now trivial to select the
  appropriate one). On Win64 the appropriate system functions are _stati64(),
  etc.
- add _pystat_fromstructstat(), it builds the return tuple for the fstat
  system call. This functionality was being duplicated. As well the
  construction of the tuple was modified to ensure no overflow of the time_t
  elements (sizeof(time_t) > sizeof(long) on Win64).
- add overflow protection for the return values of posix_spawnv and
  posix_spawnve
- use the proper 64-bit capable lseek() on Win64
- use intptr_t instead of long where appropriate from Win32/64 blocks
  (sizeof(void*) > sizeof(long) on Win64)