style.9: Add a small blurb about allowing bool

It was allowed before, but make it very explicit it is allowed now. And
prefer 'bool' to older types that were used for the same purpose -- int and
boolean_t.

Like with the C99 fixed-width types, use common sense when changing old
code.

No igor regressions.

Suggested by: bde <20151205031713.T3286@besplex.bde.org>
Reviewed by: glebius, davide, bapt (earlier versions)
Reviewed by: imp
Feedback from: julian
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D4384