Rework the EXAMPLES section to be a bit clearer.
- Add an example of using etcupdate diff.
- Create a subsection on bootstrapping that is below the simple
  examples.  This should make it clearer that 'etcupdate extract' is
  a one-time operation and not part of the common workflow.  It also
  adds more suggestions on when bootstrapping is needed and additional
  steps to make future merges simpler.

Reviewed by:	adrian
MFC after:	3 days
Replace a magic number with the proper definition.  This change actually
fixes broken state field after r273266, i.e., "CPU-1" was displayed in place
of "RUN".
Fix a clang 3.5 warning about abs(3) being given an argument of type
quad_t in setusercontext().  While here, sanitize the clamping of the
priority value, and use the correct type for the return value of
login_getcapnum().

Reviewed by:	kib
MFC after:	3 days
.a's are installed in /usr/lib, don't delete them.