Make postgres_fdw's "Relations" output agree with the rest of EXPLAIN.

The relation aliases shown in the "Relations" line for a foreign scan
didn't always agree with those used in the rest of EXPLAIN's output.
The regression test result changes appearing here provide examples.

It's really impossible for postgres_fdw to duplicate EXPLAIN's alias
assignment logic during postgresGetForeignRelSize(), because of the
de-duplication that EXPLAIN does on a global basis --- and anyway,
trying to duplicate that would be unmaintainable.  Instead, just put
numeric rangetable indexes into the string, and convert those to
table names/aliases in postgresExplainForeignScan, which does have
access to the results of ruleutils.c's alias assignment logic.
Aside from being more reliable, this shifts some work from planning
to EXPLAIN, which is a good tradeoff for performance.  (I also
changed from using StringInfo to using psprintf, which makes the
code slightly simpler and reduces its memory consumption.)

A kluge required by this solution is that we have to reverse-engineer
the rtoffset applied by setrefs.c.  If that logic ever fails
(presumably because the member tables of a join got offset by
different amounts), we'll need some more cooperation with setrefs.c
to keep things straight.  But for now, there's no need for that.

Arguably this is a back-patchable bug fix, but since this is a mostly
cosmetic issue and there have been no field complaints, I'll refrain
for now.

Discussion: https://postgr.es/m/12424.1575168015@sss.pgh.pa.us

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