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