Resubmit pull request 312

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.

- Fixes the get_memorx() function that should return both real and avail memory
- As the first array entry is now really real mem a few places had to be changed where avail mem was requested.
- Fixes FreeBSD pf configuring the table-entries limit when more than 2GB of RAM are installed [1].

[1]: https://github.com/bsdperimeter/pfsense/pull/312

I'll also look into the bogons update process to avoid the "Cannot allocate memory" errors, but that might take a few more days.