No need to return a empty string literal of the wrong const-ness when we
already have a empty string of the correct const-ness.

PR/43563: Wolfgang Solfrank: boot.cfg doesn't support comments
Fix makes it support # comments and treat spaces and tabs the same way.

ANSI and KNF from Anon Ymous

Allow parsing more than one command line switch. From anon ymous.

add new macro BOOT_FLAG() (defined in <sys/boot_flag.h>) - this
maps standard boot flags to corresponding RB_* values
use BOOT_FLAG() in port's MD code as appropriate

as discussed on tech-kern, add

add new macro BOOT_FLAG() (defined in <sys/boot_flag.h>) - this
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.

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constify first arg to parseopts()

Factor out functions for table driven boot menus and command line parsing
usable for different boot programs.