--autounmask: show note suggesting how to disable

--autounmask: show note suggesting how to disable

Now that --autounmask is enabled by default, people are dealing with
local masks that trigger unsatisfied dependencies often complain about
--autounmask behavior being inappropriate for their situation.
Therefore, show a helpful note when the autounmask code is triggered
and --quiet is not enabled:

  NOTE: This --autounmask behavior can be disabled by setting
        EMERGE_DEFAULT_OPTS="--autounmask=n" in make.conf.