Show the pid of listening process in the hello line.

* src/assuan-listen.c (assuan_accept): Use getpid instead of
assuan_get_pid.

This changes the initial interaction. Before the change, the PID of
peer was shown from listening process (but not used at all, in the
code). That is, it was the PID of connecting process. After the
change, the PID of listening process is shown, which can be used by
connecting process.

• GnuPG-bug-id: T6236
• Fixes-commit: a00b1ede5403f1e38534cc3b232739f20b8567e9
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>