pgsql: Fix pg_basebackup so that it accepts 0 as a valid compression le

Fix pg_basebackup so that it accepts 0 as a valid compression level.

The help message for pg_basebackup specifies that the numbers 0 through 9
are accepted as valid values of -Z option. But, previously -Z 0 was rejected
as an invalid compression level.

Per discussion, it's better to make pg_basebackup treat 0 as valid
compression level meaning no compression, like pg_dump.

Back-patch to all supported versions.

Reported-By: Jeff Janes
Reviewed-By: Amit Kapila
Discussion: CAMkU=1x+GwjSayc57v6w87ij6iRGFWt=hVfM0B64b1_bPVKRqg(at)mail(dot)gmail(dot)com

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/013f423729a8e7b75bbb80db5212fe320a146343

Modified Files
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doc/src/sgml/ref/pg_basebackup.sgml | 2 +-
src/bin/pg_basebackup/pg_basebackup.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)