MFC r273837:

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 273644,273738:
Clarify that pthread_cleanup_push()/pop() are implemented as macros that
create a new code block and thus must be balanced at the same lexical
scope.  (This is also a requirement in POSIX.)

PR:		194280
Submitted by:	dr2867.business@pacbell.net
MFC 273834:
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.