postgres_fdw: possible overlooked NAMEDATALEN comparison in stats import

Hi,

I’m testing "[28972b6fc] Add support for importing statistics from remote servers". Functionally, so far so good. I just found a piece of suspicious code that might be an oversight in the follow-up commit aa1f93a3387:
```
/*
 * Compare two RemoteAttributeMappings for sorting.
 */
static int
remattrmap_cmp(const void *v1, const void *v2)
{
	const RemoteAttributeMapping *r1 = v1;
	const RemoteAttributeMapping *r2 = v2;

	return strncmp(r1->remote_attname, r2->remote_attname, NAMEDATALEN);
}
```  

This function compares remote attribute names only up to NAMEDATALEN. But aa1f93a3387 changed remote_attname from a NAMEDATALEN-byte array to a char pointer, and its commit message explicitly says "the remote column name in particular could be longer than NAMEDATALEN - 1". In that case, remattrmap_cmp() could treat two distinct remote column names as equal for sorting, while match_attrmap() later compares the full strings.

Also, aa1f93a3387’s commit message mentions match_attrmap(), and that function uses strcmp(). So I guess remattrmap_cmp() should also use strcmp().

I don’t have a repro that triggers a bad result. I just want to report and confirm whether that code should be changed. I made a local change as the attached diff, and no test is broken with the change.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/