Fix Voicemail mailboxes, message types, and durations

[mattt]

This PR follows up on #253 with fixes from trying the Voicemail service on a real store:

• voicemails_fetch reads the inbox by default. A new mailbox parameter returns blocked or deleted messages, and each result names its mailbox.
• Message types come from ZPROVIDER and ZMESSAGETYPE instead of the file extension. Some carrier voicemails are stored as m4a, and FaceTime video messages can be lowercase mov.
• When ZDURATION is 0, the duration comes from the file's movie header.
• Phone numbers in the sender filter match by digits, ignoring formatting.
• id accepts an integer as well as a string.
• The service is green, to match its icon, and the server instructions list it.

It also removes the voicemail store tests from the CLI test target. They'll move with the database access into a standalone package.