In moving this function from e2e-test, I forgot that it depends on a
variable set there.

This test is properly gated on tmux 2.4.

This reverts commit aa96fc86.

tmux doesn't support BCE until 2.4, so this change fails on 2.3.

Log connection change events to syslog in the auth log, logging the PID,
username and remote host.

Also log session begin and end.

Co-Authored-By: John Hood <cgull@glup.org>

Implement true color support define in:
https://en.wikipedia.org/wiki/ANSI_escape_code

The sequence is:
ESC[ … 38;2;;; … m Select RGB foreground color
ESC[ … 48;2;;; … m Select RGB background color

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

This helps catch Perl syntax/version/dependency issues at build time
rather than run time.

A couple of them got improved out of existence.

(revert assert(false) added by 4835dcf5)
(fixes #918)

These cause warnings from static checkers, and seem to lead to spurious
GCC7 -Wmaybe-uninitialized.

There are many more.

Some of these are large and contain smaller unrelated refactors.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

This slows terminal emulation slightly.