postgres_fdw: Replace buffers in RemoteAttributeMapping with pointers.

postgres_fdw: Replace buffers in RemoteAttributeMapping with pointers.

Commit 28972b6fc ("Add support for importing statistics from remote
servers.") stored the names of local/remote columns for a foreign table
into the buffers of NAMEDATALEN bytes in this structure, without
accounting for the possibility that the remote column name in particular
could be longer than NAMEDATALEN - 1.  If it was longer than that, this
would leave it unterminated/truncated in the buffer, invoking undefined
behavior when match_attrmap() processes it, which assumes that it's
fully-contained/terminated in the buffer.

To fix, replace the buffers with char pointers, pstrdup the local/remote
column names, and store the results into the pointers.  This commit also
adds a function to clean up the nested data structure.

Per Coverity and Tom Lane.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Author: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Etsuro Fujita <etsuro.fujita@gmail.com>
Discussion: https://postgr.es/m/342868.1776017700%40sss.pgh.pa.us