Fix bug in pg_basebackup -F plain -R.

Fix bug in pg_basebackup -F plain -R.

Commit caba97a9d9 changed pg_basebackup -F plain -R so that
it overwrote postgresql.auto.conf in the backup, with new connection
setting. This could cause the existing postgresql.auto.conf settings
in the server to get lost unexpectedly. This is a bug.

This commit fixes the bug by making pg_basebackup -F plain -R
append the connection setting into postgresql.auto.conf in the backup.

Author: Fujii Masao
Reviewed-by: Sergei Kornilov
Discussion: http://postgr.es/m/250dcf2a-94e7-c05e-824a-73cfb38a48a4@oss.nttdata.com