session: override LANG with user configured language

We need to set LANG based on whats in accountsservice, for the
language configuration in control-center to work.

This commit partially reverts commit

c672e4e49983d285e829c1ec846d22fbf19bf29a

https://bugzilla.gnome.org/show_bug.cgi?id=730478
worker: really don't block SIGUSR1

The manager process alters the signal mask, so we need to reset it to
the expected default value when launching the session as the signal
mask is inherited from the parent.

https://bugzilla.gnome.org/show_bug.cgi?id=731228