nsenter: Provide an option to join target process's socket net namespace

nsenter: Provide an option to join target process's socket net namespace

The network namespace of a socket can be different from the target
process. Previously there were some userspace issues where a
net-namespace was held alive by a socket leak. For this purpose Arista's
linux kernel has a patch to provide socket => netns map by procfs pid/fd
directory links.

Add nsenter option to join the network namespace of a target process'
socket.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>