units: Tell login to preserve environment (#6023)

units: Tell login to preserve environment (#6023)

Make agetty started by *getty* units pass '-p' option to "login", so it
doesn't clear the environment and passes whatever was setup by systemd
to shells. This is needed especially for programs which are specified as
user shells, but won't read locale settings from anywhere but
environment.

[zj: cherry-pick just the second patch from the series, see discussion
on the pull request.]