socket-proxyd: rename from saproxy

socket-proxyd: rename from saproxy

The thing is a daemon, hence needs a "d" prefix. Also, we tend to not
abbreviate names of background components unnecessarily, since they are
not primary commands people type. Then, the fact that this thing does
socket actviation is mostly in implementationd detail for the proxy.

Also, do some minor indenting clean-ups and other code updates.