Do a bit of renaming as we better differentiate the CHERI system class from
general CHERI class mechanisms:

- Use the string 'cheri_system_user' for appliation extensions to the system
  class, rather than 'cheri_enter_user', and avoid unnecessarily using the
  string 'enter' in other places.

- Rename 'cheri_enter_getsandbox' to 'cheri_getsandbox', since it is not
  strictly linked to the idea of 'entering' a method.

- Update tcpdump to reflect these changes.
Add two new errno values, EPROT (for a memory protection failure) and
ENOMETHOD (when a method number is requested but is not matched).
First cut at implementing a new system class, cheri_fd, which wraps file
descriptors with CHERI object capabilities.  This will be our first use of
per-object data capabilities, as the previous system object was a singleton.