Fix assuan_get_pid for pipe servers.

At some point in the past we introduced a regression in that the
client of a pipe server received its own pid and not the pid of
the server.

• src/assuan-pipe-connect.c (struct at_pipe_fork)

(struct at_socketpair_fork): Add PARENT_PID.
(at_pipe_fork_cb, at_socketpair_fork): Use PARENT_PID instead of getpid.
(pipe_connect, socketpair_connect): Set PARENT_PID.