Fix handling of unsolicited [CAPABILITY xxx] report in server greeting.

Fix handling of unsolicited [CAPABILITY xxx] report in server greeting.

Fix up imapx_untagged() and imap_parse_status() to handle capabilities,
and use imapx_untagged() to read the server greeting.

Unify the greeting handling for the process and socket connections, and
don't re-fetch the capabilities immediately if we have them.

Handle incoming lines of noise before the greeting.