XMLFileBacked: always watch xincludes

In Bcfg2 1.3, XInclude'd files only got FAM watches if the parent file
was monitoring itself, and it got a FAM object passed to it.  This led
to some ugly workarounds, and bugs -- you couldn't easily use XInclude
in Properties or Bundler, for instance, and have changes recognized.
Since FAM objects are now module-level, we can always monitor
XInclude'd files.
Metadata: Clean up FAM monitoring cruft

Removed code that supported an unused watch_clients option that
permitted locally disabling FAM monitoring for Metadata; and ensured
that XIncludes from clients.xml/groups.xml were always monitored
(fixed in 5b66845).