- Add correct systemd requires (boo#976922)

- Update to 2.2
+ Incompatible changes:
* The format strings which referenced time have been removed. Instead:
#{t:window_activity} can be used.
* Support for TMPDIR has been removed. Use TMUX_TMPDIR instead.
* UTF8 detection how happens automatically if the client supports it, hence
the mouse-utf8, utf8 options has been removed.
* The mouse_utf8_flag format string has been removed.
* The -I option to show-messages has been removed. See #{t:start_time}
format option instead.
+ Other changes:
* Panes are unzoomed with selectp -LRUD
* New formats added:
#{scroll_position}
#{socket_path}
#{=10:...} -- limit to N characters (from the start)
#{=-10:...} -- limit to N characters (from the end)
#{t:...} -- used to format time-based formats
#{b:...} -- used to ascertain basename from string
#{d:...} -- used to ascertain dirname from string
#{s:...} -- used to perform substitutions on a string
* Job output is run via the format system, so formats work again
* If display-time is set to 0, then the indicators wait for a key to be
pressed.
* list-keys and list-commands can be run without starting the tmux server.
* kill-session learns -C to clear all alerts in all windows of the session.
* Support for hooks (internal for now), but hooks for the following have been
implemented: