Clear connection input buffer when closing

Clear connection input buffer when closing

An RFC-violating reply from the Dovecot SMTP submission service
uncovered a bug in Mutt.  Mutt was not clearing out the input buffer
when closing the connection.

Dovecot was including an extra 250 response after data submission:

  DATA
  354 OK
  Subject: Test 3

  Test 3
  .
  250 2.0.0  223 byte chunk, total 223
  250 2.0.0 OK id=1iIqT1-0004wS-Ac
  quit
  221 2.0.0 Bye

The multiline 250 reply requires a hyphen in the first response.  Mutt
closes the connection after the quit, but ends up leaving the
unexpected second line in the input buffer.  This causes an error in
the next usage of the connection.

Clean out bufpos and available when closing the socket.

Co-authored-by: Richard Russon <rich@flatcap.org>