Move _bsnmptools_debug extern from bsnmpmap.c to bsnmptools.h

It was used in bsnmpmap.c but was stored in bsnmptools.c; moving the extern
to the header allows us to cover all of our bases for the variable, and allows
_bsnmptools_debug to be used in the future elsewhere -- not just bsnmpmap.c.

MFC after: 3 weeks
Sponsored by: EMC / Isilon Storage Division