respect urlinclude/urlexclude of other web special remotes

respect urlinclude/urlexclude of other web special remotes

When a web special remote does not have urlinclude/urlexclude
configured, make it respect the configuration of other web special
remotes and avoid using urls that match the config of another.

Note that the other web special remote does not have to be enabled.
That seems ok, it would have been extra work to check for only ones that
are enabled.

The implementation does mean that the web special remote re-parses
its own config once at startup, as well as re-parsing the configs of any
other web special remotes. This should be a very small slowdown
unless there are lots of web special remotes.

Sponsored-by: Dartmouth College's DANDI project