[Pm-utils] Tiny little patches to aid in debugging

[Victor Lowther]

This changeset introduces two features targeted at those of us who develop
and debug pm-utils.

The first introduces a new environment variable PM_DEBUG.  If this variable
is exported set to true, every script which sources functions will be traced 
with -x.

The second introduces test-pm-utils which automates the process of building
and installing pm-utils in a scratch directory.  test-pm-utils has not been
added to any of the automake machinery, and should not exist in a production
environment if it is added to automake.

Since both of these changes are tiny and have been well-tested in the process
of POSIXifying the scripts and making them relocatable, and have no effect
in a production environment, I have gone ahead and pushed them to the fd.o
repository.