Systemd: parse environment variables where only value is quoted

Fixes regression introduced in 5226ae7, which added support to parse fully
quoted "NAME=value" environment vars.  To support both, this commit stores
the quotes in the value if it's only the value quoted.

Fixes RHBZ#1138508

(cherry picked from commit f64d8bc7a7670f3af2549fdcefb64c2b5f22cd0d)