Move src/bin/pg_verifybackup/parse_manifest.c into src/common.

Move src/bin/pg_verifybackup/parse_manifest.c into src/common.

This makes it possible for the code to be easily reused by other
client-side tools, and/or by the server.

Patch by me. Review of this patch in particular by at least Peter
Eisentraut; reviewers for the patch series in general include Dilip
Kumar, Andres Fruend, David Steele, Álvaro Herrera, and Jakub Wartak.

Discussion: http://postgr.es/m/CA+TgmoZ6UGZVnSy5iak6s6+AXu_DewXovDjhLs3-su6nmU_x_g@mail.gmail.com