cli: Better error message for too long socket path
cli: Better error message for too long socket path

This verify the socket path length according to
http://man7.org/linux/man-pages/man7/unix.7.html.

The proxy fails correctly when the socket path is too long, but the
message is too generic:
```
init: couldn't create AF_UNIX socket:
listen unix SOCKET_PATH: bind: invalid argument
```
Fixes #78

Signed-off-by: Jose Carlos Venegas Munoz <821ab150128e889cb3ad9aab6e72689fd9ad26f2@intel.com>

clearcontainers/proxy,clearcontainers/proxy