libpfctl: allow access to the fd

pfctl_open() opens both /dev/pf and a netlink socket. Allow access to the /dev/
pf fd via pfctl_fd().
This means that libpfctl users no longer have to open /dev/pf themselves for any
calls that are not yet available in libpfctl.

Sponsored by: Rubicon Communications, LLC ("Netgate")
MFC after: 2 weeks