Use Postfix mydomain as SRS_DOMAIN default value

postsrsd.init.in

 . /lib/lsb/init-functions
 
 # Default configuration
-SRS_DOMAIN=`[ ! -r /etc/mailname ] || cat /etc/mailname | cut -d. -f2-`
+SRS_DOMAIN=`postconf -h mydomain || true`
 SRS_FORWARD_PORT=10001
 SRS_REVERSE_PORT=10002
 SRS_SECRET=/etc/@PROJECT_NAME@.secret
 
 POSTSRS_OPTS="-4 -f$SRS_FORWARD_PORT -r$SRS_REVERSE_PORT -d$SRS_DOMAIN -s$SRS_SECRET -u$RUN_AS -p$PIDFILE -D"
 
-test -r "$SRS_SECRET" || exit 0
+test -r "$SRS_SECRET" -a -n "$SRS_DOMAIN" || exit 0
 
 ret=0
 case "$1" in