... into a new file, fe_utils/recovery_gen.c.

This can later be used by pg_rewind.

Authors: Paul Guo, Jimmy Yih, Ashwin Agrawal.  A few tweaks by Álvaro Herrera
Reviewed-by: Michaël Paquier
Discussion: https://postgr.es/m/CAEET0ZEffUkXc48pg2iqARQgGRYDiiVxDu+yYek_bTwJF+q=Uw@mail.gmail.com