maps standard boot flags to corresponding RB_* values
use BOOT_FLAG() in port's MD code as appropriate
as discussed on tech-kern, add new boot flags -v, -q for booting
verbosely or quietly, and corresponding AB_VERBOSE/AB_QUIET
boot flags; also add FreeBSD-compatible bootverbose macro and
NetBSD-specific bootquiet macro
for hpcmips, use new bootverbose instead of it's own hpcmips_verbose
Tested on i386, and to limited extend (compile of affected files) also for
mvme68k, hp300, luna68k, sun3.
