Add ability to filter events by priority/cleanups

Clean up the handling of priority levels within rules. It used to be a
mix of strings handled in various places. Now, in falco_common.h there's
a consistent type for priority-as-number as well as a list of
priority-as-string values. Priorities are passed around as numbers
instead of strings. It's still permissive about capitalization.

Also add the ability to load rules by severity. New falco
config option "priority=<val>"/-o priority=<val> specifies the minimum
priority level of rules that will be loaded.

Add unit tests for same. The test suppresses INFO notifications for a
rule/trace file combination that would otherwise generate them.

userspace/engine/falco_engine.h

@@ -67,10 +67,13 @@ public:
 	// Wrapper that assumes the default ruleset
 	void enable_rule_by_tag(const std::set<std::string> &tags, bool enabled);
 
+	// Only load rules having this priority or more severe.
+	void set_min_priority(falco_common::priority_type priority);
+
 	struct rule_result {
 		sinsp_evt *evt;
 		std::string rule;
-		std::string priority;
+		falco_common::priority_type priority_num;
 		std::string format;
 	};
 
@@ -158,6 +161,7 @@ private:
 	uint16_t m_next_ruleset_id;
 	std::map<string, uint16_t> m_known_rulesets;
 	std::unique_ptr<sinsp_evttype_filter> m_evttype_filter;
+	falco_common::priority_type m_min_priority;
 
 	//
 	// Here's how the sampling ratio and multiplier influence