byobu-shell: respect ~/.hushlogin file

According to `man login`:
"You may turn off the printing of the system message file, /etc/motd, by creating a zero-length file .hushlogin in your login directory."

The presence of this file is ignored by byobu-shell and the motd is printed regardless of the user's preference.

This was fine and I didn't even notice on Ubuntu since the motd contained useful dynamic information, but I've just installed 5.80 on a FreeBSD-based machine and the motd's just a small wall of text (granted it specifically tells me to edit /etc/motd, but it would be nice if I didn't have to). I know the FreeBSD port is several versions behind, but comparing the relevant code suggests there is still no handling of ~/.hushlogin in the latest version.