Add errdetail() with PID and UID about source of termination signal.

Add errdetail() with PID and UID about source of termination signal.

When a backend is terminated via pg_terminate_backend() or an external
SIGTERM, the error message now includes the sender's PID and UID as
errdetail, making it easier to identify the source of unexpected
terminations in multi-user environments.

On platforms that support SA_SIGINFO (Linux, FreeBSD, and most modern
Unix systems), the signal handler captures si_pid and si_uid from the
siginfo_t structure.  On platforms without SA_SIGINFO, the detail is
simply omitted.

Author: Jakub Wartak <jakub.wartak@enterprisedb.com>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Chao Li <1356863904@qq.com>
Discussion: https://postgr.es/m/CAKZiRmyrOWovZSdixpLd3PGMQXuQL_zw2Ght5XhHCkQ1uDsxjw@mail.gmail.com