libgssapi: modernize static string array use

Use designated initializers to document positions in the arrays rather
than requiring counting. Use nitems() rather than rolling it by hand to
count elements.

Also, passify a Clang 12 warning about suspcious string concatenation
within an array initializer by adding parentheses.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D26592