libgssapi: modernize static string array use

[brooks]

Summary

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.

[emaste]

lib/libgssapi/gss_display_status.c

134

If I were king of style(9) I would take an over-80-cols line vs relying on string concatenation; parens here make me stop and think for a second before accepting that the meaning is clear. Since I'm not the king of style, no objection.