core/service: serialize and deserialize accept_socket

core/service: serialize and deserialize accept_socket

This fixes an issue during reexec — the count of connections would be lost:

[zbyszek@fedora-rawhide ~]$ systemctl status testlimit.socket | grep Connected
 Accepted: 1; Connected: 1

[zbyszek@fedora-rawhide ~]$ sudo systemctl daemon-reexec
[zbyszek@fedora-rawhide ~]$ systemctl status testlimit.socket | grep Connected
 Accepted: 1; Connected: 0

With the patch, Connected count is preserved.

Also add "Accept Socket" to the dump output for services.