Move some DBM functions to Qpsmptd::DB::File::DBM

Not everything is moved and Qpsmtpd::DB* does not yet have test coverage
Get rid of experimental 'use lib' thing
Move the rest of DBM operations to Qpsmtpd::DB
Use Qpsmtpd::DB in karma plugin

And clean up a few things in Qpsmtpd::DB
Init Qpsmtpd::DB in karma plugin

And remove old DBM subs
Remove unnecessary variable
Move karma_tool to Qpsmtpd::DB

Tested manually with karma_tool which has no test coverage
Merge pull request #160 from jaredj/abstract-dbm

Move DBM functions to Qpsmtpd::DB