Get rid of mmd->filename.

Get rid of mmd->filename.

The info is also contained in afd, but this was a pointer to data in
the shared memory area. We must copy into the static mmd buffer anyway,
since com_stat() must be able to see it. So make afd.path a static
array rather than a pointer and use afd->path instead of mmd->filename.
This simplifies save_afd() and load_afd() a bit since now only the
chunk table is of variable length.