This commit adds build flags for the various architectures to prevent
failures when cross compiling. It also fixes errors in
netns_unspecified.go by replacing the undefined type Namespace with
NsHandle
Signed-off-by: Dave Tucker <dt@docker.com>
GetFromPid gets the namespace of the main thread, so previously Get
returned the wrong value if run from any other thread. Add a new
GetFromThread which uses the Linux thread id and switch Get to it.