libpfctl: support flushing rules/nat/eth

Move the code to flush regular rules, nat rules and Ethernet rules into
libpfctl for easier re-use.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D34442

(cherry picked from commit f0c334e4deca08b56b4226539c844a7018e9182d)