MFC r332421: vt: add three more cp437 mappings for vga textmode

MFC r332421: vt: add three more cp437 mappings for vga textmode

In UTF-8 locales mandoc uses a number of characters outside of the Basic
Latin group, e.g. from general punctuation or miscellaneous mathematical
symbols, and these rendered as ? in text mode.

This change adds (char, replacement, code point, description):

    – - U+2013 En Dash
    ⟨ < U+27E8 Mathematical Left Angle Bracket
    ⟩ > U+27E9 Mathematical Right Angle Bracket

This change addresses some common cases; there are others that still
need to be added after a more thorough review.

PR:		227409
Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/10@332644 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f