plugin: Add "not set" plugin value type

This value is used as a sentinel to determine if the configured
eve.json filetype was located.

First, the built-in and internal output plugins are checked. If the
sentinel value remains set, the external plugins are searched for the
filetype.

src/util-logopenfile.h

@@ -33,12 +33,15 @@
 
 #include "suricata-plugin.h"
 
-enum LogFileType { LOGFILE_TYPE_FILE,
-                   LOGFILE_TYPE_SYSLOG,
-                   LOGFILE_TYPE_UNIX_DGRAM,
-                   LOGFILE_TYPE_UNIX_STREAM,
-                   LOGFILE_TYPE_REDIS,
-                   LOGFILE_TYPE_PLUGIN };
+enum LogFileType {
+    LOGFILE_TYPE_FILE,
+    LOGFILE_TYPE_SYSLOG,
+    LOGFILE_TYPE_UNIX_DGRAM,
+    LOGFILE_TYPE_UNIX_STREAM,
+    LOGFILE_TYPE_REDIS,
+    LOGFILE_TYPE_PLUGIN,
+    LOGFILE_TYPE_NOTSET
+};
 
 typedef struct SyslogSetup_ {
     int alert_syslog_level;