vt: add 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+00A6 Broken bar
✓ √ U+2713 Checkmark

Sponsored by: The FreeBSD Foundation