Fix configuration cache invalidation

Addresses several issues with configuration cache invalidation.
The symptom of these problems was that the cache was not
invalidated when it should have been.

Specifically:

Fixes an inconsistency between bash and other shell in which
the -ot test behaves differently when the left hand file does
not exist.

Fixes an issue with host name change detection on Mac OS.

Fixes an issue detect that dcache.conf was added.

Target: trunk
Request: 2.7
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6293/
Ticket: http://rt.dcache.org/Ticket/Display.html?id=8131