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.

Author: hlinnaka

src/bin/pg_dump/Makefile

@@ -20,7 +20,7 @@ override CPPFLAGS := -I$(libpq_srcdir) $(CPPFLAGS)
 
 OBJS=	pg_backup_archiver.o pg_backup_db.o pg_backup_custom.o \
 	pg_backup_files.o pg_backup_null.o pg_backup_tar.o \
-	dumputils.o $(WIN32RES)
+	dumputils.o compress_io.o $(WIN32RES)
 
 KEYWRDOBJS = keywords.o kwlookup.o