Implement CalDAV/Card Service Discovery via SRV records (RFC 6764)

[rfc2822]

This will allow services that provide SRV records for CalDAV and CardDAV to be detected automatically by domain name.

For instance, if a user enters "icloud.com" as the host name, the corresponding services can be detected:

$ host -t SRV _caldavs._tcp.icloud.com
_caldavs._tcp.icloud.com has SRV record 0 0 443 caldav.icloud.com.
$ host -t SRV _carddavs._tcp.icloud.com
_carddavs._tcp.icloud.com has SRV record 0 0 443 contacts.icloud.com.

The advantage is that when CalDAV and CardDAV services are on different servers (like on iCloud), they can be used in one account when SRV services discovery is used.

[rfc2822]

A quick investigation which services provide CalDAV/CardDAV SRV records (alphabetically):

• fastmail.fm
• fruux.com
• icloud.com
• mailbox.org
• mykolab.com
• yandex.ru

[rfc2822]

Implemented with and 2f5622edaf84a15f239c6bdaa483ca3da40b8958 and 2e6a3efd253a986cf177f812d47b64f0e611f1e0.