Windows: fix mlock, remove ftruncate and fix error handling.

Windows: fix mlock, remove ftruncate and fix error handling.

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 <axboe@kernel.dk>