Rather like "arg0path='/foo/'" matching all object paths starting with
"/foo/", this adds support for matching a prefix of a string argument
with "arg0namespace='org.freedesktop.Telepathy.Client.'" (for example).

This is mostly intended for use with NameOwnerChanged and
PropertiesChanged; thus, only matching the 0th argument is permitted.
(This also means it could work with the multicast-plus-socket-filters
model being considered for DBus-in-the-kernel without having to hash
every period-separated prefix of every string argument.)