units: make graphical.target dependencies more complete and similar to those of multi-user.target

units: drop [Install] section from multi-user.target and graphical.target They were supposed to make it easy to make the default.target a symlink to these targets, but this was never advertised and we have a better command for this now in "systemctl set-default". Since the install section makes the output of "systemctl list-unit-files" confusing (since it makes the units appear as "disabled"), let's drop the sections.

units: add explicit dependency on display-manager.service to graphical.target This replaces the symlink based dependency by an explicit one in the unit file so that we avoid the dangling symlink when no display manager is installed.

units: introduce new Documentation= field and make use of it everywhere This should help making the boot process a bit easier to explore and understand for the administrator. The simple idea is that "systemctl status" now shows a link to documentation alongside the other status and decriptionary information of a service. This patch adds the necessary fields to all our shipped units if we have proper documentation for them.

relicense to LGPLv2.1 (with exceptions) We finally got the OK from all contributors with non-trivial commits to relicense systemd from GPL2+ to LGPL2.1+. Some udev bits continue to be GPL2+ for now, but we are looking into relicensing them too, to allow free copy/paste of all code within systemd. The bits that used to be MIT continue to be MIT. The big benefit of the relicensing is that closed source code may now link against libsystemd-login.so and friends.

units: we no longer need m4 to build graphical.target or multi-user.taregt