[BUG] npm pack should not attempt to include unix socket files

[coreyfarrell]

How

See https://github.com/coreyfarrell/pack-socket for demonstration

Current Behavior

If a unix socket file has a filename that would be included in npm pack it is not ignored. This causes a cb() never called! error.

Steps to Reproduce

On Linux (or maybe OSX):

git clone https://github.com/coreyfarrell/pack-socket
cd pack-socket
npm pack

Expected Behavior

Unix socket files should be ignored regardless of .npmignore / package.json#files settings