This change was originally done for the next commit, to fix browser
completion handling.  However, I discovered the browser does its own
relative path expansion, and could just be fixed by using the _norel()
version.

Still, I think this change is a good idea in any case.  There may be a
few more fixes needed that require relpath expansion while keeping
trailing slashes.

Since the number of flag parameters to expand_path would become
excessive by adding a "remove_trailing_slash" paremeter, convert it to
use a single 'flags' parameter instead.