Merge branch 'master' into core-updates

gnu: tmux: Update to 1.9a.

* gnu/packages/tmux.scm (tmux): Update to 1.9a

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

Merge branch 'master' into core-updates

Conflicts:
	gnu-system.am

gnu: Adjust synopses as recommended by 'guix lint'.

Remove leading articles and trailing periods in package synopses.

gnu: Add tmux.  licenses: add ISC.

* gnu/packages/tmux.scm: New file.
* Makefile.am (MODULES): Use it.
* guix/licenses.scm: add ISC.