Improve notation of cacheinfo table in syscache.c.

Improve notation of cacheinfo table in syscache.c.

Use C99 designated initializer syntax for the array elements, instead of
writing the enumerator name and position in a comment.  Replace nkeys
and key with a local variadic macro, for a shorter notation.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/CA%2BhUKGKdpDjKL2jgC-GpoL4DGZU1YPqnOFHbDqFkfRQcPaR5DQ%40mail.gmail.com