hgwebdir: read --webdir-conf as actual configuration to ui (issue1586)
This cleans up code and allows specification of values more globally. For
example, it's now possible to specify web.contact in webdir-conf for all
repositories without a specified contact set.
% basic
% unknown
*** [alias] unknown: command bargle is unknown
% ambiguous
*** [alias] ambiguous: command s is ambiguous
% recursive
*** [alias] recursive: circular dependency on recursive
adding foo
% with opts
C foo