Allow ALTER SYSTEM SET on unrecognized custom GUCs

Currently we have this odd behavior (for a superuser):

regression=# ALTER SYSTEM SET foo.bar TO 'baz';
ERROR: unrecognized configuration parameter "foo.bar"
regression=# SET foo.bar TO 'baz';
SET
regression=# ALTER SYSTEM SET foo.bar TO 'baz';
ALTER SYSTEM

That is, you can't ALTER SYSTEM SET a random custom GUC unless there
is already a placeholder GUC for it, because the find_option call in
AlterSystemSetConfigFile fails. This is surely pretty inconsistent.
Either the first ALTER SYSTEM SET ought to succeed, or the second one
ought to fail too, because we don't have any more knowledge about the
custom GUC than we did before.

In the original discussion about this [1], I initially leaned towards
"they should both fail", but I reconsidered: there doesn't seem to be
any harm in allowing ALTER SYSTEM SET to succeed for any custom GUC
name, as long as you're superuser.

Hence, attached is a patch for that. Much of it is refactoring to
avoid duplicating the code that checks for a reserved GUC name, which
I think should still be done here --- otherwise, we're losing a lot of
the typo detection that that check was intended to provide. (That is,
if you have loaded an extension that defines "foo" as a prefix, we
should honor the extension's opinion about whether "foo.bar" is
valid.) I also fixed the code for GRANT ON PARAMETER so that it
follows the same rules and throws the same errors for invalid cases.

There's a chunk of AlterSystemSetConfigFile that now needs indenting
one more tab stop, but I didn't do that yet for ease of review.

Thoughts?

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/169746329791.169914.16613647309012285391%40wrigleys.postgresql.org