Export some more useful info about shared memory objects to userland
via procstat(1) and fstat(1):
- Change shm file descriptors to track the pathname they are associated
  with and add a shm_path() method to copy the path out to a caller-supplied
  buffer.
- Use the fo_stat() method of shared memory objects and shm_path() to
  export the path, mode, and size of a shared memory object via
  struct kinfo_file.
- Add a struct shmstat to the libprocstat(3) interface along with a
  procstat_get_shm_info() to export the mode and size of a shared memory
  object.
- Change procstat to always print out the path for a given object if it
  is valid.
- Teach fstat about shared memory objects and to display their path,
  mode, and size.

capability mode and capabilities.

Right now no attempt is made to unwrap capabilities when operating on
a crashdump, so further refinement is required.

Approved by:	re (bz)
Sponsored by:	Google Inc

  file and processes information retrieval from the running kernel via sysctl
  in the form of new library, libprocstat.  The library also supports KVM backend
  for analyzing memory crash dumps.  Both procstat(1) and fstat(1) utilities have
  been modified to take advantage of the library (as the bonus point the fstat(1)
  utility no longer need superuser privileges to operate), and the procstat(1)
  utility is now able to display information from memory dumps as well.

  The newly introduced fuser(1) utility also uses this library and able to operate
  via sysctl and kvm backends.

  The library is by no means complete (e.g. KVM backend is missing vnode name
  resolution routines, and there're no manpages for the library itself) so I
  plan to improve it further.  I'm commiting it so it will get wider exposure
  and review.

  We won't be able to MFC this work as it relies on changes in HEAD, which
  was introduced some time ago, that break kernel ABI.  OTOH we may be able
  to merge the library with KVM backend if we really need it there.

Discussed with:	rwatson