__opendkim: fix start_on_boot on FreeBSD

There was a bit of an oddity with this, it is implemented in a way
that should not be an issue for other systems.

type/__opendkim/manifest

@@ -29,6 +29,7 @@ case "$os" in
 'freebsd')
 	CFG_DIR="/usr/local/etc/mail"
 	service="milter-opendkim"
+	start_service="milteropendkim"
 	;;
 *)
 	printf "__opendkim does not yet support %s.\n" "$os" >&2
@@ -90,7 +91,7 @@ fi
 require="__package/opendkim" __file "$target_file" \
 	--source "$source_file" --mode 0644
 
-require="__package/opendkim" __start_on_boot "${service}"
+require="__package/opendkim" __start_on_boot "${start_service:-${service}}"
 
 # Ensure Key and Signing tables exist and have proper permissions
 key_table="${CFG_DIR}/KeyTable"
@@ -105,7 +106,7 @@ require="__package/opendkim" \
 	--mode 444
 
 require="__file${target_file} __file${key_table}
-		__file${signing_table} __start_on_boot/${service}" \
+		__file${signing_table} __start_on_boot/${start_service:-${service}}" \
 	__check_messages opendkim \
 		--pattern "^__file${target_file}" \
 		--execute "service ${service} restart"