Add nsenter --config option #65

Docker-DCO-1.1-Signed-off-by: Dhammika Pathirana <dhammika@gmail.com> (github: dhammika)

Patch to add --config <config_file_path> option for large args. 
Also fixed possible SIGSEG crash on realloc() failure, it's unlikely to happen with mem overcommit on.
But then some systems run without overcommit and realloc() can fail under real OOM conditions.
I did bit of testing with files and named pipes, probably needs more testing. 
I'm guessing we're doing this separation to eventually send all args via FD? Is that something we want to do? The only long arg is the container config and if anything things are moving more and more there.

Also, we don't need to read from that FD here, that can be done in the Go code that gets run after this. Hopefully, this will only have to enter the namespaces.