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.
and some people may use shells which do not support it. Instead, make an empty
default-command option mean a login shell, and fork it with a - in argv[0]
which is the method used by login(1).
Also fix the automatic-rename code to handle this correctly and to strip a
leading - if present.
terminal to be switched between several different windows and programs
displayed on one terminal be detached from one terminal and moved to another.
ok deraadt pirofti
