bash completion: Allow completions to work without external functions

If bash_completion wasn't installed, _filedir wouldn't be defined which
led to all filename-based completions to error out. Specifically
autocompletion would fail when a filename was expected and when
bash_completion wasn't installed. Now we fall back to `compgen -f` if
_filedir fails. According to _filedir's comments, compgen doesn't
handle files with spaces well, but it is still better to complete most
files than none.