Disallow setting fillfactor for TOAST tables.

To implement this without almost duplicating the reloption table, treat
relopt_kind as a bitmask instead of an integer value.  This decreases the
range of allowed values, but it's not clear that there's need for that much
values anyway.

This patch also makes heap_reloptions explicitly a no-op for relation kinds
other than heap and TOAST tables.

Patch by ITAGAKI Takahiro with minor edits from me.  (In particular I removed
the bit about adding relation kind to an error message, which I intend to
commit separately.)