ccm-purge: add support for selectable database backends

ccm-purge was still assuming that the profile data was being stored in
GDBM. This updates the call to use the correct EDG::WP4::CCM::DB
indirection to handle all backends.

This fixes #14 on github.
ccm-purge: fix whitespace

Replace all tabs with spaces.
ccm-purge: Keep minimum number of old profiles

Add a new parameter to ccm.conf that controls how many old profiles
ccm-purge will keep. These are kept even if their last access time is
older than the aging time. Also switch ccm-purge to match the
documentation by checking atime not mtime.

Due to the way ccm-fetch parses ccm.conf with AppConfig the parameter
also had to be added to ccm-fetch.
POD fixes for ccm-fetch
Make purge time configurable via ccm.conf

Adds a variable, purge_time, to ccm.conf. Resolves #15.

Note, due to the way ccm-fetch uses AppConfig, ccm-fetch also gains a
--purge_time argument which is ignored.
Use LC::Stat constants in ccm-purge

The previous attempt to convert ccm-purge to use atime instead of mtime
of the old profiles update the wrong part of the code. Having tested
updating the correct part of the code, it seems that ccm-purge can
modify the atime of the profile directory which is perhaps why it was
using mtime. The use of mod_time as the variable name also suggeststhat
perhaps the documentation was wrong not the code.

Updated the documentation to reflect the use of mtime and also
introduced the use of LC::Stat's constants to make it more readable.
Replace ${file} with $file in an error message

maven was replacing ${file} with the name of the pom.xml used to
generate the package. $file works just as well in this context and is
not touched by maven.
Merge pull request #16 from ned21/ccm-purge-fixes

CCM purge fixes