Check to be sure file exists before reading and include flowbits rule file if missing in snort.conf

Change Log
================================================
January 23, 2013

1.  Add checks for existence of rules files prior to attempting
     to read them to prevent errors during intial startup of Snort
     following a remove and re-install (or after initial install).

2.  Add a check in the automated rules update function to 
     ensure the flowbit rules file is included in snort.conf if the 
     auto-flowbit resolution feature is enabled.  This addresses 
     a problem during initial Rules Update after install of Snort 
     where rules directory is empty and thus the snort.conf 
     file does not include any rules files.

3.  Added similar checks for missing rules files in the RULES 
     and CATEGORIES tabs to prevent errors after initial 
     Snort install but prior to downloading the first set of rules.

These fixes address the issues reported by users on the 
pfSense forums after the last release of the Snort 2.5.3
package.