Merge pull request #1869 from poettering/kill-overridable

Remove support for RequiresOverridable= and RequisiteOverridable=

diff --cc TODO
Simple merge

diff --cc man/systemctl.xml
Simple merge

diff --cc man/systemd.unit.xml
Simple merge

diff --cc src/core/manager.c
Simple merge

diff --cc src/core/unit.c
Simple merge

diff --cc src/shared/generator.c
Simple merge

diff --cc src/systemctl/systemctl.c
Simple merge