logging: refactor enabling of debug messages

Reorder the LOGS_Severity enum in order of severity and change the code
to not log/print messages with severity below the specified minimum
instead of having a separate debug level.

logging.h

@@ -31,9 +31,6 @@
 
 #include "sysincl.h"
 
-/* Flag indicating whether debug messages are logged */
-extern int log_debug_enabled;
-
 /* Line logging macros.  If the compiler is GNU C, we take advantage of
    being able to get the function name also. */
 
@@ -55,7 +52,7 @@ extern int log_debug_enabled;
 
 #define DEBUG_LOG(...) \
   do { \
-    if (DEBUG && log_debug_enabled) \
+    if (DEBUG && log_min_severity == LOGS_DEBUG) \
       LOG_MESSAGE(LOGS_DEBUG, __VA_ARGS__); \
   } while (0)
 
@@ -69,13 +66,16 @@ extern int log_debug_enabled;
 
 /* Definition of severity */
 typedef enum {
-  LOGS_INFO,
+  LOGS_DEBUG = -1,
+  LOGS_INFO = 0,
   LOGS_WARN,
   LOGS_ERR,
   LOGS_FATAL,
-  LOGS_DEBUG
 } LOG_Severity;
 
+/* Minimum severity of messages to be logged */
+extern LOG_Severity log_min_severity;
+
 /* Init function */
 extern void LOG_Initialise(void);
 
@@ -92,12 +92,10 @@ FORMAT_ATTRIBUTE_PRINTF(2, 3)
 extern void LOG_Message(LOG_Severity severity, const char *format, ...);
 #endif
 
-/* Set debug level:
-   0, 1 - only non-debug messages are logged
-   2    - debug messages are logged too, all messages are prefixed with
-          filename, line, and function name
-   */
-extern void LOG_SetDebugLevel(int level);
+/* Set the minimum severity of a message to be logged or printed to terminal.
+   If the severity is LOGS_DEBUG and DEBUG is enabled, all messages will be
+   prefixed with the filename, line number, and function name. */
+extern void LOG_SetMinSeverity(LOG_Severity severity);
 
 /* Log messages to a file instead of stderr, or stderr again if NULL */
 extern void LOG_OpenFileLog(const char *log_file);