MFC r327444, r327449, r327454

r327444:
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
Differential Revision: https://reviews.freebsd.org/D13645

r327449:
Unbreak build broken by r327444

During review iterations function signature has changed in definition
but not in actual call. Fix call to match the definition.

Reported by: Herbert J. Skuhra
Pointyhat to: gonzo

r327454:
Fix GCC build broken by r32744

Indicate in function declaration that vt_palette_init does not take any arguments