dnsmasq: add 'extraconftext' parameter

Users can now freely add new dnsmasq parameters (i.e. a whole config)
via extraconf. This means users can add their own parameters without
changes to init or GUI.

Co-opted the default of confdir also to include the instance name.
This way each instance gets its own .d directory (and separate instances
do not all inherit the same 'extraconftext').

Usage:
config dnsmasq 'config'
	...
	option extraconftext 'cache-size=2048\nlog-async=20'

config dnsmasq 'blah'
	...
	option extraconftext 'cache-size=128\nlog-async=5'

or even (which would produce staggered output but still valid)

config dnsmasq 'blah'
	...
	option extraconftext 'cache-size=128
							log-async=5'

See https://forum.openwrt.org/t/add-dnsmasq-custom-options-field-in-luci-gui/193184

Tested on: 23.05.3, 22.03.6

Signed-off-by: Paul Donald <[email protected]>
Tested-by: Vladimir Kochkovski <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/14975
Signed-off-by: Hauke Mehrtens <[email protected]>

package/network/services/dnsmasq/files/dnsmasq.init

@@ -12,6 +12,7 @@ ADD_WAN_FQDN=0
 ADD_LOCAL_FQDN=""
 
 BASECONFIGFILE="/var/etc/dnsmasq.conf"
+EXTRACONFFILE="extraconfig.conf"
 BASEHOSTFILE="/tmp/hosts/dhcp"
 TRUSTANCHORSFILE="/usr/share/dnsmasq/trust-anchors.conf"
 TIMEVALIDFILE="/var/state/dnsmasqsec"
@@ -1121,14 +1122,19 @@ dnsmasq_start()
 	xappend "--dhcp-broadcast=tag:needs-broadcast"
 
 
-	config_get dnsmasqconfdir "$cfg" confdir "/tmp/dnsmasq.d"
+	# Create a dnsmasq.d dir for each instance
+	config_get dnsmasqconfdir "$cfg" confdir "/tmp/dnsmasq${cfg:+.$cfg}.d"
 	xappend "--conf-dir=$dnsmasqconfdir"
 	dnsmasqconfdir="${dnsmasqconfdir%%,*}"
-	[ ! -d "$dnsmasqconfdir" ] && mkdir -p $dnsmasqconfdir
+	[ ! -d "$dnsmasqconfdir" ] && mkdir -p "$dnsmasqconfdir"
 	xappend "--user=dnsmasq"
 	xappend "--group=dnsmasq"
 	echo >> $CONFIGFILE_TMP
 
+	# EXTRACONFFILE allows new dnsmasq parameters before they are natively handled in this init file
+	config_get extraconftext "$cfg" extraconftext
+	[ -n "$extraconftext" ] && echo -e "$extraconftext" > "$dnsmasqconfdir"/"$EXTRACONFFILE"
+
 	config_get_bool enable_tftp "$cfg" enable_tftp 0
 	[ "$enable_tftp" -gt 0 ] && {
 		config_get tftp_root "$cfg" tftp_root