libpfctl: implement status counter accessor functions

The new nvlist-based status call allows us to easily add new counters.
However, the libpfctl interface defines a TAILQ, so it's not quite
trivial to find the counter consumers are interested in.

Provide convenience functions to access the counters.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D41649

(cherry picked from commit e3d3d61a7d94a4155ef70048a8b578985fca8383)