When using tmux as a login shell, there is currently no way to specify a shell
to be used as a login shell inside tmux, so add a default-shell session option.
This sets the shell invoked as a login shell when the default-command option is
empty.
The default option value is whichever of $SHELL, getpwuid(getuid())'s pw_shell
or /bin/sh is valid first.
Based on a diff from martynas@, changed by me to be a session option rather
than a window option.
by reading argv[0] from the process group leader of the group that owns the tty
(tcgetpgrp()). This can't be done portably so some OS-dependent code is
introduced (ugh); OpenBSD, FreeBSD and Linux are supported at the moment.
A new window flag, automatic-rename, is available: if this is set to off, the
window name is not changed. Specifying a name with the new-window, new-session
or rename-window commands will automatically set this flag to off for the
window in question. To disable it entirely set the option to off globally (setw
-g automatic-rename off).