Log Message:
tmux: Update misc/tmux to 2.6

Changes:
CHANGES FROM 2.5 TO 2.6, 05 October 2017
* Add select-pane -T to set pane title.
* Fix memory leak when lines with BCE are removed from history.
* Fix (again) the "prefer unattached" behaviour of attach-session.
* Reorder how keys are checked to allow keys to be specified that have a
  leading escape. GitHub issue 1048.
* Support REP escape sequence (\033[b).
* Run alert hooks based on options rather than always, and allow further bells
  even if there is an existing bell.
* Add -d flag to display-panes to override display-panes-time.
* Add selection_present format when in copy mode (allows key bindings that do
  something different if there is a selection).
* Add pane_at_left, pane_at_right, pane_at_top and pane_at_bottom formats.
* Make bell, activity and silence alerting more consistent by: removing the
  bell-on-alert option; adding activity-action and silence-action options with
  the same possible values as the existing bell-action; adding a \ 
"both" value
  for the visual-bell, visual-activity and visual-silence options to trigger
  both a bell and a message.
* Add a pane_pipe format to show if pipe-pane is active.
* Block signals between forking and resetting signal handlers so that the
  libevent signal handler doesn't get called in the child and incorrectly write
  into the signal pipe that it still shares with the parent. GitHub issue 1001.
* Allow punctuation in pane_current_command.
* Add -c for respawn-pane and respawn-window.
* Wait for any remaining data to flush when a pane is closed while pipe-pane is
  in use.
* Fix working out current client with no target. GitHub issue 995.
* Try to fallback to C.UTF-8 as well as en_US.UTF-8 when looking for a UTF-8
  locale.
* Add user-keys option for user-defined key escape sequences (mapped to User0
  to User999 keys).
* Add pane-set-clipboard hook.
* FAQ file has moved out of repository to online.
* Fix problem with high CPU usage when a client dies unexpectedly. GitHub issue
  941.
* Do a dance on OS X 10.10 and above to return tmux to the user namespace,
  allowing access to the clipboard.
* Do not allow escape sequences which expect a specific terminator (APC, DSC,
  OSC) to wait for forever - use a small timeout. This reduces the chance of
  the pane locking up completely when sent garbage (cat /dev/random or
  similar).
* Support SIGUSR2 to toggle logging on a running server, also generate the
  "out" log file with -vv not -vvvv.
* Make set-clipboard a three state option: on (tmux both sends to outside
  terminal and accepts from applications inside); external (tmux sends outside
  but does not accept inside); and off.
* Fix OSC 4 palette setting for bright foreground colours. GitHub issue 954.
* Use setrgbf and setrgbb terminfo(5) capabilities to set RGB colours, if they
  are available. (Tc is still supported as well.)
* Fix redrawing panes when they are resized several times but end up with the
  size unchanged (for example, splitw/resizep -Z/breakp).
* Major rewrite of choose mode. Now includes preview, sorting, searching and
  tagging; commands that can be executed directly from the mode (for example,
  to delete one or more buffers); and filtering in tree mode.
* choose-window and choose-session are now aliases of choose-tree (in the
  command-alias option).
* Support OSC 10 and OSC 11 to set foreground and background colours.
* Check the U8 capability to determine whether to use UTF-8 line drawing
  characters for ACS.
* Some missing notifications for layout changes.
* Control mode clients now do not affect session sizes until they issue
  refresh-client -C. new-session -x and -y works with control clients even if
  the session is not detached.
* All new sessions that are unattached (whether with -d or started with no
  terminal) are now created with size 80 x 24. Whether the status line is on or
  off does not affect the size of new sessions until they are attached.
* Expand formats in option names and add -F flag to expand them in option values.
* Remember the search string for a pane even if copy mode is exited and entered
  again.
* Some further BCE fixes (scroll up, reverse index).
* Improvements to how terminals are cleared (entirely or partially).