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. 
LONG_MIN/LONG_MAX check was for int underflow/overflow checks, I think it's better to check this with errno. But we still need init_pid <= 0 if pid is specified as a negative.