Disallow "=" in names of reloptions and foreign-data options.

Disallow "=" in names of reloptions and foreign-data options.

We store values for these options as array elements with the syntax
"name=value", hence a name containing "=" confuses matters when
it's time to read the array back in.  Since validation of the
options is often done (long) after this conversion to array format,
that leads to confusing and off-point error messages.  We can
improve matters by rejecting names containing "=" up-front.

(Probably a better design would have involved pairs of array
elements, but it's too late now --- and anyway, there's no
evident use-case for option names like this.  We already
reject such names in some other contexts such as GUCs.)

Reported-by: Chapman Flack <jcflack@acm.org>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Chapman Flack <jcflack@acm.org>
Discussion: https://postgr.es/m/6830EB30.8090904@acm.org
Backpatch-through: 13