Split the server code handling clients, jobs and windows off into separate
files from server.c (merging server-msg.c into the client file) and rather than
iterating over each set after poll(), allow a callback to be specified when the
fd is added and just walk once over the returned pollfds calling each callback
where needed.

More to come, getting this in so it is tested.

Don't try to unsuspend a client if it isn't suspended.

Support for individual session idle time locking. May be enabled by turning off
the lock-server option (it is on by default). When this is off, each session
locks when it has been idle for the lock-after-time setting. When on, the
entire server locks when ALL sessions have been idle for their individual
lock-after-time settings.

This replaces one global-only option (lock-after-time) with another
(lock-server), but the default behaviour is usually preferable so there don't
seem to be many alternatives.

Diff/idea largely from Thomas Adam, tweaked by me.

Don't allow locked or suspended clients to limit the size of active clients.

Support -c like sh(1) to execute a command, useful when tmux is a login
shell. Suggested by halex@.

This includes another protocol version increase (the last for now) so again
restart the tmux server before upgrading.

Remove the internal tmux locking and instead detach each client and run the
command specified by a new option "lock-command" (by default "lock -np") in
each client.

This means each terminal has to be unlocked individually but simplifies the
code and allows the system password to be used to unlock.

Note that the set-password command is gone, so it will need to be removed from
configuration files, and the -U command line flag has been removed.

This is the third protocol version change so again it is best to stop the tmux
server before upgrading.

Trim some code by moving the ioctl(TIOCGWINSZ) after SIGWINCH from the client
into the server.

This is another (the second of four) protocol version changes coming this
morning, so again the server should be killed before upgrading.

Don't attempt to open() the tty path, rely on the client sending its stdin fd
with imsg and fatal if it doesn't, then set the FD_CLOEXEC flag in tty_init
instead of tty_open to prevent them leaking into child processes if any are
created between the two calls.

This bumps the protocol version, so the tmux server should be killed before
upgrading.

The default terminal size should be 80x24, not 80x25.