Read /etc/default/networking when started from systemd. Closes: #806883

Add an undocumented --read-environment option which will cause the
VERBOSE, CONFIGURE_INTERFACES and EXCLUDE_INTERFACES environment
variables to be read and act in the same way as with ifupdown's SysV init
script. This allows the use of EnvironmentFile=-/etc/default/networking
in the systemd service file.