changed the size of a char array in the userdata_struct from 1 to 16

changed the size of a char array in the userdata_struct from 1 to 16
to account for padding/alignment issues. Eventually I'd like to find a
way to get rid of this construct altogether as it is a bit error
prone and hard to debug.

also added a new macro:

ZERO_STRUCTP() that takes a pointer to a structure and zeros the
structure. Used in nmbd to zero allocated structures before freeing
them to try to catch bugs a bit faster.