Add pg_rewind, for re-synchronizing a master server after failback.

Earlier versions of this tool were available (and still are) on github.

Thanks to Michael Paquier, Alvaro Herrera, Peter Eisentraut, Amit Kapila,
and Satoshi Nagayasu for review.

doc/src/sgml/reference.sgml

@@ -260,6 +260,7 @@
    &pgControldata;
    &pgCtl;
    &pgResetxlog;
+   &pgRewind;
    &postgres;
    &postmaster;