fzf-tmux temporarily turns off remain-on-exit and synchronize-panes
options. We don't have to try to restore the values of the options if
they were already turned off when fzf-tmux was started.

Restore the original values of 'remain-on-exit' and 'synchronize-panes'
options when exiting 'fzf-tmux'.

Close #1039

Close #1007

Close #935

Fix #796

Using a dummy command that works as the barrier.

Fix #702

Close #699

Fixes #643
Doesn't break #648

This reverts commit 4d444777.

Avoids confusing grouped sessions.
Fixes #643

For consistency and (negligible) performance improvement

e.g. fzf-tmux -q '$PATH'

Related: #343

Close #583

This reverts commit f074709f.

Close #586. /cc @akashin

Patch submitted by @netei. Close #524.

Related: https://github.com/junegunn/fzf.vim/issues/67

Fix #305. ncurses can crash on invalid $TERM. fzf-tmux uses bash on
a new pane so we have to make sure that the $TERM is consistent with
that of the hosting shell.

Close #303