Only show signal-sender PID/UID detail in server log

Only show signal-sender PID/UID detail in server log

The errdetail() added in 55890a91945 (and reworked in 3e2a1496bae)
exposed the operating-system PID and UID of whoever sent the
termination signal directly to the affected client.

Discussion suggested this should not be sent to the client, but only
recorded in the server log where the admin can use it for diagnosis.

Author: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.com>
Discussion: https://postgr.es/m/E5CA274C-74BD-4067-8B73-A3AD8C080EFA@gmail.com