Fixed bug in layman.list_local

If no overlays had been added, layman.list_local would fail with
IndexError
Updated layman.add and layman.delete to adjust make.conf

When the first overlay is added, the make.conf needs to source layman's
make.conf so that emerge sees the packages in the overlay. Likewise,
when the last overlay is deleted, the make.conf needs to no longer
source layman's make.conf as it will cause emerge to break.
Merge pull request #3160 from shadowfax-chc/layman

Layman bug fixes