Revert "emerge: Implement "--alert""

This reverts commit 1a00ddf5fb089724a127c8dc72d63f10ae72abbf. It had
some furiously annoying mistakes in the commit message. Also, someone
alluded to the man page being too short. Thanks, Brian.

Signed-off-by: Alexander Berntsen <bernalex@gentoo.org>
emerge: Implement "--alert"

Implement "--alert" (short option "-A"), which adds a terminal bell
character ('\a') to all prompts, when it is set to True or "y".

The patch refactors userquery to a class, UserQuery, to accommodate
--alert. It also adds the option to the emerge man page.

Signed-off-by: Alexander Berntsen <bernalex@gentoo.org>