dnsmasq: add 'extraconftext' parameter

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 <newtwen+github@gmail.com>
Tested-by: Vladimir Kochkovski <ask@getvladimir.com>
Link: https://github.com/openwrt/openwrt/pull/14975
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>