libcasper/services/cap_net: Add cap_getprotobyname

[freqlabs]

Summary

This allows for access to /etc/protocols as needed by getprotobyname in a rights-restricted process.

[oshogbo]

The whole limit infrastructure is missing for capnet,
I think I prefer the cap_netdb for those purpose.
If I recall it we had a few ideas what to add to netdb.