Fix: Correctly determine pf table-entries limit

This patch sets the table-entries limit as intended by pf with 100000 entries for systems with less than 100MB of RAM and 200000 otherwise. Due to limitations in FreeBSD the internal mechanism in pf is broken for systems with more than 2GB of RAM.

- Fix get_memorx() function which in most cases only returned avail memory instead of both real and avail memory
- In connection with that fix correct the code which made use of the avail memory
- Fix FreeBSD pf configuring the table-entries limit when more than 2GB of RAM are installed [1].

[1]: https://github.com/bsdperimeter/pfsense/pull/312
[2]: http://forum.pfsense.org/index.php/topic,57286.0.html