DNSSD Browser UI Service Type helper function: _tel._tcp

[adam-burns]

helper function for_tel._tcp, useful for browser based UIs to resolve and connect to service type.

should map URL schema to tel: (note no forward slashes)
take number from a TXT attribute, "id" ie id=+491704552266
and map into format tel:

this well known mapping will work for any browser on any platform that has a schema handler defined for tel:
mobile phones (android & ios) auto-associate this with their phone caller app (and parhaps skype & others if installed)

example: tel:+491704552266

For any browsing domain that wishes to define one or more _tel._tcp service type instances, a PTR record of (eg)

_services._dns-sd._udp.<browsing domain> PTR _tel._tcp.<browsing domain>

must exist.

For any browsing domain that has has no _tel._tcp service types existing or last remaining has been deleted, then PTR record of

_services._dns-sd._udp.<browsing domain> PTR _tel._tcp.<browsing domain>

should be deleted.

this is useful, for example, in giving contact numbers in map location points