new option to allow pg_rewind to run without full_page_writes

Hello,

Currently pg_rewind refuses to run if full_page_writes is off. This is
to prevent it to run into a torn page during operation.

This is usually a good call, but some file systems like ZFS are
naturally immune to torn page (maybe btrfs too, but I don't know for
sure for this one).

Having the option to use pg_rewind without the cost associated with
full_page_writes when using a system immune to torn page is beneficial:
increased performance and more compact WAL.

This patch adds a new option "--no-ensure-full-page-writes" to pg_rewind
for this situation, as well as patched documentation.

Regards,
Jeremie Grauer