scripts/msmtpq: only try to enqueue if not connected
scripts/msmtpq: remove debugging from the nm-online check
scripts/msmtpq: correct logging in sendmail mode

In sendmail mode, msmtpq should not log anything to standard out, as this (not
just the return code) is used to indicate failure.  Logging anything tricks,
among other things, emacs mu4e / sendmail-mode into thinking there has been a
failure.