Add DEBUG_PRINTF to stand.hHEADmain

stand/ is mostly debugged with printfs, in an ad hoc and sometimes
fragile manner. For example BOOTP_DEBUG in bootp.c cannot be defined
unless NETIF_DEBUG is defined in dev_net.c or build fails for lack of the
symbol debug.

The DEBUG_PRINTF implementation in stand.h addresses that and allows
for more control over debug output.  It is compatible with the
usage in libsecureboot.

Simply define _DEBUG_LEVEL to the desired level of debug
or in the case of libsecureboot _DEBUG_LEVEL_VAR to the variable that
will hold that value - default is _debug which is static so each
translation unit can be controlled independently.

The 1st arg to DEBUG_PRINTF is a level which must be greater than or
equal to _DEBUG_LEVEL_VAR if the printf is to be called.
See libsecureboot for more examples.

Reviewed by:	imp
Sponsored by:	Juniper Networks, Inc.
Differential Revision:	https://reviews.freebsd.org/D51269

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