loader: support numeric and named bright colors (8-15)

Accept "bright" or "light" prefix for named colors.

For numeric colors, update error message to specify that values 0 to 15
are allowed, and verify that values are in that range.

Reviewed by: imp, tsoome (both earlier version)
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34512

(cherry picked from commit 425e57e7a2f754f7be8425bf3b00ce1469aba5b0)