acticate: rename --environment to --setenv

Nspawn has --setenv, and systemd itself accepts systemd.setenv.
It is nice to have the same parameter name everywhere.

Old name is accepted, but not advertised.
systemd-python: fix setting of exception codes

The return value of 0 would be treated as failure by mistake,
resulting in " SystemError: error return without exception set".
The way that set_error() is used is changed to be the same
everywhere.
systemd-python: fix listen_fds under Python 2
man: expand on some more subtle points in systemd.socket(5)

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=727708#1694