suzerain: map for header_shown values

These grew like weeds.  Rip them out and use a single map.
Slightly slower, but no one's going to care about string
construction and map lookup costs vs parallel FFTs.

Minor attempts made to not look up keys repeatedly.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44313 0bd85f0b-c151-43b9-a1e9-27231104551f