Read and cache client socket SMACK label

Signed-off-by: Shane Bryan <shane.bryan@intel.com>
Smack: clean up this code

- use a #define for the smack label thing we're using twice
- smack label retrieval is done when packets arrive, not before
- actually store cred info in the client struct
- check return value of malloc()
- unwind code and terminate if problems exist
- free client resources manually
Make 'client' local, rename it.
Rename new_client to cl, ->credentials to ->cred

Mostly reduces the tediously long and awkward names here.