agent: Close parent pipes after container process is started
agent: Close parent pipes after container process is started

We don't need to maintain parent pipes opened after the process has
been started since libcontainer will take care of duplicating them.

More than that, leaving those pipes ends opened would mean that our
loops listening onto stdout and stderr from the process would never
get an error while reading, leading to a deadlock.

This commit solves an issue we have been having with exec. We were
sometimes missing some output because the pipes were forcibly closed
at the end of the process, instead of waiting for the read functions
to disconnect automatically.

Fixes #207

Signed-off-by: Sebastien Boeuf <90b401417360b1924ef3f520ddee932686110f85@intel.com>

clearcontainers/agent,clearcontainers/agent