namespaces: nsenter: ignore nonexistent namespaces

If a particular kernel doesn't have namespace directories that
libcontainer tries to attain during nsenter(), ignore the error (this is
consistent with the previous implementation of nsenter()).

Docker-DCO-1.1-Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> (github: cyphar)
remove unnecessary headers

Docker-DCO-1.1-Signed-off-by: Takahiro Maebashi <maebashi@iij.ad.jp> (github: maebashi)