style.9: Add a C++ section

This section adds several style guidelines for C++ in FreeBSD's base
system both enumerating some differences relative to the C KNF style
and addressing some unique C++ idioms not addressed by the existing
KNF style.

This section is not exhaustive but does include an initial set of
guidelines.

Reviewed by: ivy, emaste
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D50983