vt(4): add support for configurable console palette

Introduce new set of loader tunables kern.vt.color.N.rgb, where N is a
number from 0 to 15. The value is either comma-separated list decimal
numbers ranging from 0 to 255 that represent values of red, green, and
blue components respectively (i.e. "128,128,128") or 6-digit hex triplet
commonly used to represent colors in HTML or xterm settings (i.e. #808080)

Each tunable overrides one of the 16 hardcoded palette codes and can be set
in loader.conf(5)

Reviewed by: bcr(docs), jilles, manu, ray
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D13645