Add netlink hooks to delete a bridge dev

Signed-off-by: Dhammika Pathirana <dhammika@gmail.com>

Docker-DCO-1.1-Signed-off-by: Dhammika Pathirana <dhammika@gmail.com> (github: dhammika)
Add code review updates

Docker-DCO-1.1-Signed-off-by: Dhammika Pathirana <dhammika@gmail.com> (github: dhammika)
Reopening stdin, stdout and stderr if they are pointing to /dev/null to not have '/dev/null' from the global namespace opened inside the container.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Initially mount /sys as ro instead of remount

The issue with doing a remount as ro with sysfs is that if a container
is still in one of the hosts namepsaces, commonly with the NET
namespace, the remount will cause the host's systems sysfs to be
remounted as ro also.  We can fix this correctly by not doing the
remount and just mount sys as ro in the first place.

The other remounts are individual files within proc so they will not
have this issue.

For context please see:
https://github.com/dotcloud/docker/issues/7101
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Merge pull request #110 from crosbymichael/sysro

Initially mount /sys as ro instead of remount
Fix 386 and arm cross-compile

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #111 from tianon/fix-cross

Fix 386 and arm cross-compile
Add linux/386 testing back to Travis

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #112 from tianon/travis-cross-again

Add linux/386 testing back to Travis
Add more Travis matrix targets (being explicit about CGO)

I've also updated the comments with the current status of the commented-out targets (since we're getting close to having them available for our Travis-use in the relevant upstreams).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #113 from tianon/travis-cgo

Add more Travis matrix targets (being explicit about CGO)
Merge pull request #46 from dhammika/44-delete-bridge

Add netlink hooks to delete a bridge dev #44
Merge pull request #107 from vishh/dev_null

Reopening stdin, stdout and stderr if they are pointing to /dev/null.
Small fix for GetAllCgroups().

Incorrect parsing of length of fields.

Docker-DCO-1.1-Signed-off-by: Victor Marmol <vmarmol@google.com> (github: vmarmol)
Merge pull request #114 from vmarmol/cgroups

Small fix for GetAllCgroups().
Add "linux/arm" to Travis

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #115 from tianon/travis-arm

Add "linux/arm" to Travis