pg_rewind enhancements

Hi,

While using pg_rewind, I found that it is a bit difficult to use pg_rewind
as it seems to copy even the configuration files and also remove some of
the files created on the old primary which may not be present on the new
primary. Similarly it copies files under the data directory of the new
primary which may not be needed or which possibly could be junk files.

I would propose to have a couple of new command line arguments to
pg_rewind. One, a comma separated list of files which should be preserved
on the old primary, in other words which shouldn't be overwritten from the
new primary. Second, a comma separated list of files which should be
excluded while copying files from the new primary onto the old primary.

Would like to invite more thoughts from the hackers.

Regards,
RKN