Add the ability to SPAMFILTER message-tags.

The new target type is called 'T' and we match against "name=value"
of each message tag (or just "name" if it is without value).

Example: SPAMFILTER ADD -simple T kill 0 this_is_a_test +typing=active
(No this is not a suggestion :D)

This probably won't be used much at all, but it is good to have the
option available in case there is some massive problem,
especially since more message tags may pop up sooner or later.

Caveat: this is actually a bit slow as we may have to check multiple
message tags for a single line.
If there are zero message-tag spamfilters then we will automatically
short-circuit and save all this CPU, which will be the most common case.

include/struct.h

@@ -992,12 +992,13 @@ struct Secret {
 #define SPAMF_USERMSG		0x0002 /* p */
 #define SPAMF_USERNOTICE	0x0004 /* n */
 #define SPAMF_CHANNOTICE	0x0008 /* N */
-#define SPAMF_PART			0x0010 /* P */
-#define SPAMF_QUIT			0x0020 /* q */
-#define SPAMF_DCC			0x0040 /* d */
-#define SPAMF_USER			0x0080 /* u */
-#define SPAMF_AWAY			0x0100 /* a */
-#define SPAMF_TOPIC			0x0200 /* t */
+#define SPAMF_PART		0x0010 /* P */
+#define SPAMF_QUIT		0x0020 /* q */
+#define SPAMF_DCC		0x0040 /* d */
+#define SPAMF_USER		0x0080 /* u */
+#define SPAMF_AWAY		0x0100 /* a */
+#define SPAMF_TOPIC		0x0200 /* t */
+#define SPAMF_MTAG		0x0400 /* m */
 
 /* Other flags only for function calls: */
 #define SPAMFLAG_NOWARN		0x0001