Move config file parsing from tpkg executable to tpkg.rb

[jheiss]

Config file parsing should be done in the library rather than the executable so that all users of the library benefit from config file support.

Additionally the path for finding tpkg.conf should respect the file_system_root option to allow for unit testing of the config file parsing. I added this to etch recently and it could serve as an example.

Was: https://sourceforge.net/apps/trac/tpkg/ticket/28