pg_rewind: Skip log directory for file type check like pg_wal

Hello hackers,

I think we should extend the "log" directory the same courtesy as was
done for pg_wal (pg_xlog) in 0e42397f42b.

Today, even if BOTH source and target servers have symlinked "log"
directories, pg_rewind fails with:

file "log" is of different type in source and target.

Attached is a repro patch using the 004_pg_xlog_symlink.pl test to
demonstrate the failure.
Running make check PROVE_TESTS='t/004_pg_xlog_symlink.pl'
in src/bin/pg_rewind should suffice after applying.

This is because when we use the libpq query to fetch the filemap from
the source server, we consider the log directory as a directory, even if
it is a symlink. This is because pg_stat_file() is used in that query in
libpq_traverse_files() and pg_stat_file() returns isdir=t for symlinks
to directories.

This shortcoming is somewhat called out:

* XXX: There is no backend function to get a symbolic link's target in
* general, so if the admin has put any custom symbolic links in the data
* directory, they won't be copied correctly.

We could fix the query and/or pg_stat_file(). However, we would also
like to support deployments where only one of the primaries and/or
standbys have the symlink. That is not hard to conceive, given primaries
and standbys can have drastically disparate log volume and/or log
collection requirements.

Attached is a patch that treats "log" like we treat "pg_wal".

Regards,
Soumyadeep (VMware)