layman.py: Adds api variant of new() function

Also cleans up output messages as well as some variable
interpolation.
Renames sync dir and plugin name to "layman_"

This change has been made to prevent loading the wrong layman
modules and causing conflicts when using the portage sync plugin.
layman_.py: Adds re-initialization prevention code

This commit prevents unnecessary re-initialization of the LaymanAPI
in the layman sync plugin module.