auth: dovecot: pass remote IP (rip=) to auth server

If known, let the auth server know where the client came
from, using REMOTE_ADDR or, optionally/configurably, the
X-Remote-Addr header value (which is needed when running
behind a trusted proxy.)

Addresses #1859.

config

@@ -136,6 +136,9 @@
 # Port of via network exposed dovecot socket
 #dovecot_port = 12345
 
+# Use X-Remote-Addr for remote IP (rip) in dovecot authentication
+#dovecot_rip_x_remote_addr = False
+
 # IMAP server hostname
 # Syntax: address | address:port | [address]:port | imap.server.tld
 #imap_host = localhost