Initial ebuilds for IBR DTN packages version 0.6.5.

Tested on ~amd64, more work may be required especially regarding USE
flags and dependencies (versions, slots, etc.).
Add README.
Rewrap README.
Add usage instructions to README.
Die if econf failed.
Add initscript and initscript config when libdaemon support is requested.

Therefore, also install default config and spool directory.
Create a user and group for this daemon.
Bump ebuild revision.
Update Manifest file for ibrdtnd.
Update README as overlay can now be used via layman.
Change how URLs are referenced in README, include link to ibrdtn's wiki.
Bump version of all ibrdtn packages to 0.8.0.
Update all Manifest files.
Add systemd USE flag and unit to ibrdtnd ebuild.