systemctl: show Environment entries with whitespace

systemctl: show Environment entries with whitespace

This makes the Environment entries more round-trippable: a similar format is
used for input and output. It is certainly more useful for users, because
showing [unprintable] on anything non-trivial makes systemctl show -p Environment
useless in many cases.

Fixes: #14723 and https://bugzilla.redhat.com/show_bug.cgi?id=1525593.

$ systemctl --user show -p Environment run-*.service
Environment=ASDF=asfd "SPACE= "
Environment=ASDF=asfd "SPACE=\n\n\n"
Environment=ASDF=asfd "TAB=\t\\" "FOO=X X"