Move mmap_full_file from osl.c to fd.c.

Move mmap_full_file from osl.c to fd.c.

To make it independent from osl.c, we must not pass
a pointer to struct osl_data. Replace it by void *, size_t *
pointers. Also add new int *fd_ptr parameter. If it's non-NULL
the file is not closed after mmap, and the open fd is returned
in fd_ptr.