weston-launch: Clear environment and set it up from scratch

With recent systemd[1] XDG_VTNR will leak through to pam, which ends up
setting a vtnr pam argument with the wrong value.  The fix is to clear
XDG_VTNR first, but what we should have been doing all along is resetting
the environment.

Thanks to Ray Strode for helping debug this.

[1] http://cgit.freedesktop.org/systemd/systemd/commit/?id=a8573ccc35a4efe8900be5d48c6c803670540c2b
1 file changed