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
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

sys/dev/vt/hw/vga/vt_vga.c

@@ -237,6 +237,7 @@ static const struct unicp437 cp437table[] = {
 	{ 0x03c0, 0xe3, 0x00 }, { 0x03c3, 0xe5, 0x00 },
 	{ 0x03c4, 0xe7, 0x00 }, { 0x03c6, 0xed, 0x00 },
 	{ 0x03d5, 0xed, 0x00 }, { 0x2010, 0x2d, 0x00 },
+	{ 0x2013, 0x2d, 0x00 },
 	{ 0x2014, 0x2d, 0x00 }, { 0x2018, 0x60, 0x00 },
 	{ 0x2019, 0x27, 0x00 }, { 0x201c, 0x22, 0x00 },
 	{ 0x201d, 0x22, 0x00 }, { 0x2022, 0x07, 0x00 },
@@ -285,7 +286,8 @@ static const struct unicp437 cp437table[] = {
 	{ 0x2640, 0x0c, 0x00 }, { 0x2642, 0x0b, 0x00 },
 	{ 0x2660, 0x06, 0x00 }, { 0x2663, 0x05, 0x00 },
 	{ 0x2665, 0x03, 0x01 }, { 0x266a, 0x0d, 0x00 },
-	{ 0x266c, 0x0e, 0x00 },
+	{ 0x266c, 0x0e, 0x00 }, { 0x27e8, 0x3c, 0x00 },
+	{ 0x27e9, 0x3e, 0x00 },
 };
 
 static uint8_t