fuse/impl_c: add --auto-fsid option

fuse/impl_c: add --auto-fsid option

this was inspired by reading the fuse docs and discovering the 'fsid' option
to fuse_main, and was _intended_ to support a sort of 'stability' to the
filesystem (specifically derived from the root-uri mounted, whether directly
or via an alias) to support mac aliases across unmount/remount etc.

some experimentation shows that that doesn't actually work, and that, at
least for mac aliases in my testing, they're tied to path-to-mountpoint and
not to the fsid - which seems to have no bearing.  perhaps the 'local' flag
is causing weirdness therein.

at any rate, I'm recording it simply for posterity, in case it turns out to
be useful after all somewhere down the road.