MFC 297942:
libgssapi: avoid NULL pointer dereferences.

While here also use NULL instead of zero for pointers.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Convert files to UTF-8 and add some copyright markers where missing.

Fix conflicts after heimdal-1.1 import and add build infrastructure. Import
all non-style changes made by heimdal to our own libgssapi.