object description for FDW user mappings

When commit cae565e503 introduced FDW user mappings, it used this in
getObjectDescription for them:

    appendStringInfo(&buffer, _("user mapping for %s"), usename);

This was later mostly copied (by yours truly) as object identity by
commit f8348ea32e wherein I used this:
    appendStringInfo(&buffer, "%s", usename);

As it turns out, this is wrong, because the pg_user_mapping catalog has
a two-column "primary key" which is user OID and server OID.  Therefore
it seems to me that the correct object description and identity must
include both username and server name.  I propose we change the above to
this:

    appendStringInfo(&buffer, _("user mapping for %s in server %s"), usename,
                 srv->servername);

I propose to apply the (appropriately rebased) attached patch to all
back branches.  Note in particular the wording changes in some error
messages in the foreign_data test.

--
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services