debianpkg: kill backports

debianpkg: kill backports

The debianpkg/backports system is rather complicated and actually slows
down Debian package building quite a lot since the backports/rules file
is evaluated a zillion times during a normal build.

This just folds up everything into a single Debian package build that
works on all OSes.  The only real difference that the backports stuff
was used for is switching between systemd and init.d, the latter for
Ubuntu 12.04 and 14.04.

With this, that switch is controlled by the pkg.frr.nosystemd
Build-Profile instead.  Package builds for Ubuntu 14.04 need to supply
the -Ppkg.frr.nosystemd option to dpkg-buildpackage.  (12.04 isn't
supported anymore anyway.)

Note that the update-rc.d step that was previously coded into
postinst/postrm is now handled by the dh_installinit magic.

Other than this, there were some minor build dependency differences, all
of which are now just handled as | in the central deps.

Signed-off-by: David Lamparter <equinox@diac24.net>