core: log about "systemctl kill" requests

core: log about "systemctl kill" requests

let's add informational logging about each client requested signal
sending. While we are at, let's beef up error handling/log messages in
this case quite a bit: let's log errors both to syslog and report errors
back to client.

Fixes: #17254
(cherry picked from commit d991100291b6d5b81176417a42c0ca6a17cf6ab8)