generic userid: use the new USERID api

recent changes in trunk allow us to specify the userid inside the openwrt makefile.
the info is stored int he meta data of the IPK contorl file and users are generated
by the new generic postinst trigger.

Signed-off-by: John Crispin <blogic@openwrt.org>

mail/dovecot/files/dovecot.init

@@ -7,8 +7,6 @@ STOP=75
 USE_PROCD=1
 
 start_service() {
-	user_exists dovecot 59 || user_add dovecot 59
-	group_exists dovecot 59 || group_add dovecot 59
 	mkdir -p -m 0755 /var/lib/dovecot
 	mkdir -p -m 0755 /var/run/dovecot
 	chmod 0750 /var/lib/dovecot