agent: More fix for get_client_pid for portability.
- configure.ac: Check sys/ucred.h instead of ucred.h.
- agent/command-ssh.c: Include sys/ucred.h.
It's *BSD and macOS thing.
- Fixes-commit: f7f806afa5083617f4aba02fc3b285b06a7d73d4
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>