Add MD5 checks for downloaded rulesets

[colingrady]

Add MD5 checks for downloaded rulesets:

• Community Ruleset: https://snort.org/downloads/community/md5s
• Registered LightSPD and Registered Rulesets: https://snort.org/downloads/registered/md5s
• Subscriber LightSPD and Registered Rulesets: https://snort.org/downloads/subscriber/md5s

[colingrady]

The use of these MD5 files can also help to identify the rulesets available to download. e.g. Maybe use "latest" version ruleset if there isn't one available for a later version of Snort that is running.