extended who tags

Hi Aneesh,

I send you a pull request on richacl-tools for a changeset that permits to extend the "who" tags to new values found in NFSv4, CDMI and the Scality filesystem on which I work.

The numbers used in this changeset (and upstream) are a bit arbitrary and I think a better guess would be to use a configuration file that describe the precise ABI to better handle "who" tags. It will be a pretext for a later pull request.

For the time being, this changeset prevents richacl-tools to segfault when an unknown "who" tag is encountered. Furthermore, applying an unknown tag in an ACE on an unaware filesystem should not raise unintendedly privileges.

(a quick note on the INTERACTIVE@ who: its purpose is to match users not coming from the network)

Best regards,
Guillaume Gimenez