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. 
Here we read all the args since this function executes before main(). We read them to determine whether to run this code at all (only if nsenter is specified) and in that case we enter the namespaces of the PID provided (only arg this code uses). After the Go runtime comes up, it uses argv from main() and does the rest of the setup which requires the container config (the only real other piece we pass here). I think we should have nsenter take two args: <init PID> and <FD for container config>. The first for C code, the second for Go code. WDYT?