Reloptions for table access methods

A custom table access method might want to add a new reloption to
control something specific to that table access method. Unfortunately,
if you add a new option of type RELOPT_KIND_HEAP, it will immediately
fail because of the validation that happens in fillRelOptions().

Right now, heap reloptions (e.g. FILLFACTOR) are validated in two
places: parseRelOptions() and fillRelOptions().

parseRelOptions() validates against boolRelOpts[], intRelOpts[], etc.
This validation is extensible by add_bool_reloption(), etc.

fillRelOptions() validates when filling in a struct to make sure there
aren't "leftover" options. It does this using a hard-coded parsing
table that is not extensible.

Index access methods get total control over validation of reloptions,
but that doesn't fit well with heaps, because all heaps need the
vacuum-related options.

I considered some other approaches, but they all seemed like over-
engineering, so the attached patch just passes validate=false to
fillRelOptions() for heaps. That allows custom table access methods to
just define new options of kind RELOPT_KIND_HEAP.

Regards,
    Jeff Davis