dhcp: Port the plugin to FreeBSD/macOS

This also refactors the BPF handling so it can be shared between the
dhcp and farp plugins.  The latter is adapted accordingly.

Closes strongswan/strongswan#2047

Co-authored-by: Tobias Brunner <tobias@strongswan.org>

configure.ac

@@ -1853,6 +1853,7 @@ AM_CONDITIONAL(USE_ATTR, test x$attr = xtrue)
 AM_CONDITIONAL(USE_ATTR_SQL, test x$attr_sql = xtrue)
 AM_CONDITIONAL(USE_COUNTERS, test x$counters = xtrue)
 AM_CONDITIONAL(USE_SELINUX, test x$selinux = xtrue)
+AM_CONDITIONAL(USE_PF_HANDLER, test x$dhcp = xtrue -o x$farp = xtrue)
 
 #  other options
 # ---------------