Allow initializing w/ empty hostname (=local); remote paths with spaces; FAT hints

A few minor fixes and user hints to make setup for syncing to transiently mounted USB devices with funny mountpoint names just a little smoother. FAT filesystems are pretty much a standard on these things; spaces in the names are common, and the things are almost always found locally attached. It's a use case which unison fails spectacularly badly at, to the extent of forcing a full comparison the first time on each new mount. rsync is dumber, so bitpocket seems to handle it better, given the right options.