Refactor the pg_dump zlib code from pg_backup_custom.c to a separate file,

Refactor the pg_dump zlib code from pg_backup_custom.c to a separate file,
to make it easier to reuse that code. There is no user-visible changes.

This is in preparation for the patch to add a new archive format, a directory,
to perform a custom-like dump but with each table being dumped to a separate
file (that in turn is a prerequisite for parallel pg_dump). This also makes it
easier to add new compression methods in the future, and makes the
pg_backup_custom.c code easier to read, when the compression-related code is
factored out.

Joachim Wieland, with heavy editorialization by me.