Historically, the code to build relation options has been shaped the
same way in multiple code paths by using a set of datums in input with
the options parsed with a static table which is then filled with the
option values.  This introduces a new common routine in reloptions.c to
do most of the legwork for the in-core code paths.

Author: Amit Langote
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/CA+HiwqGsoSn_uTPPYT19WrtR7oYpYtv4CdS0xuedTKiHHWuk_g@mail.gmail.com