main: allow turning on debugging from environment variable

having to pass --debug is pretty onerous for the user given the
user doesn't normally run gnome-session directly.

This commit lets users throw GNOME_SESSION_DEBUG=yes into
/etc/environment
autostart: don't set DISPLAY in autostarted apps for wayland sessions

DISPLAY is an X11-ism, and is unset for wayland.  This was leading to
log spew.

https://bugzilla.gnome.org/show_bug.cgi?id=741679
main: support GNOME_SESSION_DEBUG=0 and GNOME_SESSION_DEBUG=1

rpmatch curiously doesn't grok 0 and 1 as no and yes.  This commit
throws atoi into the mix to cover those cases.
Updated Brazilian Portuguese translation