This routine determines the Kerberos realm names for host,
filling in realmlist with a pointer to
an argv[ ] style list of names, terminated with a NULL pointer.
If host is NULL, the local host's
realms are determined.
If there are no known realms for the host, the filled-in pointer
is set to NULL.
The pointer array and strings pointed to are all in allocated
storage, and should be freed by the caller when finished.