Make SPI_fnumber() reject dropped columns.

There's basically no scenario where it's sensible for this to match
dropped columns, so put a test for dropped-ness into SPI_fnumber()
itself, and excise the test from the small number of callers that
were paying attention to the case.  (Most weren't :-(.)

In passing, normalize tests at call sites: always reject attnum <= 0
if we're disallowing system columns.  Previously there was a mixture
of "< 0" and "<= 0" tests.  This makes no practical difference since
SPI_fnumber() never returns 0, but I'm feeling pedantic today.

Also, in the places that are actually live user-facing code and not
legacy cruft, distinguish "column not found" from "can't handle
system column".

Per discussion with Jim Nasby; thi supersedes his original patch
that just changed the behavior at one call site.

Discussion: <b2de8258-c4c0-1cb8-7b97-e8538e5c975c@BlueTreble.com>

src/backend/utils/adt/tsvector_op.c

@@ -2242,6 +2242,7 @@ tsvector_update_trigger(PG_FUNCTION_ARGS, bool config_column)
 				(errcode(ERRCODE_UNDEFINED_COLUMN),
 				 errmsg("tsvector column \"%s\" does not exist",
 						trigger->tgargs[0])));
+	/* This will effectively reject system columns, so no separate test: */
 	if (!IsBinaryCoercible(SPI_gettypeid(rel->rd_att, tsvector_attr_num),
 						   TSVECTOROID))
 		ereport(ERROR,