activate: simplify/rework implementation of --setenv

activate: simplify/rework implementation of --setenv

Previous implementation is simplified by using the new helper. The new code
does more looping, but considering that it's unlikely that people set more
than a handful of variables through commandline options, this should be OK.

If a variable is specified on the command line, it overrides any automatically
set variable. Effective behaviour was already were like this, because we would
specify two variables, both would be set, and since glibc will return
the first matching entry.
('systemd-socket-activate -E TERM=FOO -l 2000 --inetd -a env' would give
'TERM=FOO TERM=xterm-256color PATH=...', and getenv("TERM") returns "FOO".)
But it's nicer to filter out any duplicate entries and only pass the intended
variable to the child process.