lighdm: Rebuild to read bash init scripts

[donald]

Change shebang of /usr/bin/lightdm-session from /bin/sh to
/bin/bash --login, so that the bash startup logic reads the
same init files as a login shell in a terminal window would do.
This is what users expect, so that, for example, applications
started from the desktop launcher have the same environment
and umask as applications started from a terminal window.

The scripts executed by the login shell include /etc/profile, so we
no longer need to call it explicitly.