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 have to check this, usually optopt is set for option flag. But I'm not sure if this is done in all cases.