Add a proper usage() function.

Suggested by and OK jmc.
OK tb

src/etc/netstart

@@ -1,10 +1,16 @@
 #!/bin/sh -
 #
-#	$OpenBSD: netstart,v 1.197 2018/03/04 10:12:26 jmc Exp $
+#	$OpenBSD: netstart,v 1.198 2018/04/28 22:38:32 rpe Exp $
 
 # Turn off Strict Bourne shell mode.
 set +o sh
 
+# Show usage of the netstart script and exit.
+usage() {
+	print -u2 "usage: ${0##*/} [[-n] interface ...]"
+	exit 1
+}
+
 # Echo file $1 to stdout. Skip comment lines and delete everything
 # after the first '#' from other lines. Strip leading and trailing
 # whitespace if IFS is set.
@@ -200,23 +206,19 @@ FUNCS_ONLY=1 . /etc/rc.d/rc.subr
 _rc_parse_conf
 
 PRINT_ONLY=false
-USAGE="usage: ${0##*/} [[-n] interface ...]"
 V4_DHCPCONF=false
 V6_AUTOCONF=false
 
 while getopts ":n" opt; do
 	case $opt in
 	n)	PRINT_ONLY=true;;
-	*)	print -u2 "$USAGE"; exit 1;;
+	*)	usage;;
 	esac
 done
 shift $((OPTIND-1))
 
 # Option -n is only supported if interface names are specified as parameters.
-if $PRINT_ONLY && (($# == 0)); then
-	print -u2 "$USAGE"
-	exit 1
-fi
+$PRINT_ONLY && (($# == 0)) && usage
 
 # Load key material for the generation of IPv6 Semantically Opaque Interface
 # Identifiers (SOII) used for link local and SLAAC addresses.