Add support for 'dohpath' to SVCB (and HTTPS)

dohpath is specfied in draft-ietf-add-svcb-dns and has a value
of 7.  It must be a relative path (start with a /), be encoded
as UTF8 and contain the variable dns ({?dns}).

bin/tests/system/checkzone/zones/good-svcb.db

@@ -24,3 +24,4 @@ svcb6		SVCB	6 . mandatory=port,alpn port=60 alpn=h3
 svcb7		SVCB	7 . mandatory=port,alpn port=60 alpn=h1,h3
 svcb8		SVCB	8 . mandatory=port,alpn port=60 alpn="h1\\,h2,h3"
 svcb9		SVCB	0 44._svbc.example.net.
+svcb10		SVCB	7 . alpn="h2,h3" dohpath=/{?dns}