changed default value of getenvint to 0. fix tests

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
use exists

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #3863 from vieux/default_getint_0

changed default value of getenvint to 0. fix tests