Restore use of zlib default compression in pg_dump directory mode.

Restore use of zlib default compression in pg_dump directory mode.

This was broken by commit 0e7e355f27302b62af3e1add93853ccd45678443 and
friends, which ignored the fact that gzopen() will treat "-1" in the
mode argument as an invalid character, which it ignores, and a flag for
compression level 1. Now, when this value is encountered no compression
level flag is passed  to gzopen, leaving it to use the zlib default.

Also, enforce the documented allowed range for pg_dump's -Z option,
namely 0 .. 9, and remove some consequently dead code from
pg_backup_tar.c.

Problem reported by Marc Mamin.

Backpatch to 9.1, like the patch that introduced the bug.