Add debug.verbose_sysinit tunable for VERBOSE_SYSINIT

VERBOSE_SYSINIT is currently an all-or-nothing option. debug.verbose_sysinit
adds an option to have the code compiled in but quiet by default so that
getting this information from a device in the field doesn't necessarily
require distributing a recompiled kernel.

Its default is VERBOSE_SYSINIT's value as defined in the kernconf. As such,
the default behavior for simply omitting or including this option is
unchanged.

MFC after: 1 week