Show the WhatsApp link with /whatsapp status

Show the WhatsApp link with /whatsapp status

The daemon opens the WhatsApp session in whatsapp/session.db and keeps a
linked device connected, so the next steps can pair, read, and send. The
line /whatsapp status in the terminal client or in Telegram tells
whether WhatsApp is not linked, linked and connected, or linked and
disconnected, with the times of the connection.

The clients handle a /whatsapp line before it reaches the agent, and the
daemon answers only the client that asked, with a notice event that the
terminal client now shows. So the lines of the later pairing, which hold
the phone number and the code, never reach the log, the model, or the
other client.

The session gives full access to the account, so the daemon refuses a
session file whose mode is not 0600, and the library logs nothing,
because its log lines hold the own phone number. The daemon opens the
session only after it holds the lock of the data directory. The library
and its SQLite driver, modernc.org/sqlite, are pure Go.

Issue: 6f69ca1