core: don't insert an extra space before each SocketBind{Allow,Deny}= item

core: don't insert an extra space before each SocketBind{Allow,Deny}= item

The extra space was actually screwing up deserialization:

~# systemd-run --wait --pipe -p SocketBindAllow=any true
Running as unit: run-u167.service
Finished with result: exit-code
Main processes terminated with: code=exited/status=234
Service runtime: 1ms
CPU time consumed: 0
~# journalctl -b -p err
...
Oct 27 16:39:15 arch systemd-executor[5983]: Failed to deserialize: Invalid argument

Let's not do that by default and introduce a simple wrapper which
inserts the space after each item only when necessary.