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_common.h

@@ -74,6 +74,22 @@ public:
 
 	void set_inspector(sinsp *inspector);
 
+        // Priority levels, as a vector of strings
+	static std::vector<std::string> priority_names;
+
+	// Same as numbers/indices into the above vector
+	enum priority_type
+	{
+		PRIORITY_EMERGENCY = 0,
+		PRIORITY_ALERT = 1,
+		PRIORITY_CRITICAL = 2,
+		PRIORITY_ERROR = 3,
+		PRIORITY_WARNING = 4,
+		PRIORITY_NOTICE = 5,
+		PRIORITY_INFORMATIONAL = 6,
+		PRIORITY_DEBUG = 7
+	};
+
 protected:
 	lua_State *m_ls;