add support for testing execin

when the test binary starts, it detects whether it should run the container
initialization code or the execin initialization code based on the suppplied
arguments. The execin initialization code is taken from docker.

also added a sample test for execin process.

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)
Merge pull request #287 from dqminh/execin-test

add support for testing execin