[PATCH 0 of 2] hgwebdir configuration cleanup

[Alexander Solovyov]

First patch makes hgwebdir more flexible and easier to extend with new options,
while second uses this ability to make hgweb usable behind another webserver.

Actually first patch contains (at the very start) two lines of code I'm not sure
about: looks like parentui, coming from commands.serve, doesn't contain any
paths. Maybe they should be removed