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

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@raphink would you mind double checking this, to make sure I haven't missed a better way to support both without putting quotes in the stored value?  I think any other solution introduces ambiguities in the put direction.