Functions in posix.c need to use win_to_posix_err to convert errors from
Windows API calls to their equivalent POSIX error code so perror etc.
work: move it into posix.c and add the prototype to posix.h.
Fix error handling so -1 is always returned on error and errno is set.
Fix mlock to work with sizes greater than about 1MB: the working set size
needs to be increased for the new allocation.
Remove ftruncate from posix.c, since MinGW already provides it.
Signed-off-by: Jens Axboe <firstname.lastname@example.org>