Add draft/multiline support with a default max-lines of 15 for known-users

and 7 for unknown-users (with max-bytes 5250 and 1500 respectively). This
allows pasting a short snippet of code, config file, text from a site, etc.

With multiline you have the guarantee that:
1) You will see the entire text with no delay between lines
2) You won't see another persons chat half-way through such a paste
3) For multiline supporting clients it is now clear that all the text
   belongs to each other, which can make selecting/copying it easier.
This basically means short snippets/pastes like that can be completely on
IRC again. No need for a pastebin for it. Though, you may still need such
a service if you are pasting more lines.

Regarding the implementation in UnrealIRCd:
* Clients without multiline get individual fallback lines (concat lines
  merged, blank lines skipped, as per spec). And we know that clients like
  weechat - which does support multiline - also shows all lines and not
  only a few plus snippet style "[.."]. That is another reason for only
  allowing 15 lines by default and not something much more. Otherwise all
  those clients would get a big wall of text, which just sucks.
* Spamfilter (also) runs on the full text of all lines together, so
  splitting a phrase across lines does not evade spamfilter.
* Fakelag: a client can send the BATCH start+PRIVMSG (or NOTICE)+BATCH end
  at full speed. We impose no fake lag there. Also, the multiline default
  max-lines and max-bytes are lower than the example class::recvq of 8000,
  so should be perfectly safe. If the entire BATCH is accepted then we
  will impose fake-lag afterwards, with a cap of 15 seconds maximum.
  If the BATCH is rejected, we impose half the fakelag plus 2sec.
* If the time between BATCH start and BATCH end is more than 15 seconds
  then the BATCH is rejected (set::multiline::batch-timeout).
* The BATCH is atomic (either you see it all, or you see none of it):
  * When the client sends it to server, it is buffered first.
  * Only after the batch close the server indicates if it is accepted
    or rejected. This has various reasons, two of them are: 1) The client
    is going to send everything in one go anyway and not wait for a
    response between each PRIVMSG, and 2) we can't do many checks in the
    buffering stage and skip those after, that would cause a TOCTOU
    problem (eg. a banned user still being able to speak).
  * If any line gets rejected due to spamfilter or other case
    (eg +c, +b ~text with block, etc etc), the entire batch is rejected
  * Locally we deliver all or nothing (as said)
  * S2S we buffer the batch as well, so if a server splits after having
    received 10 lines out of 15, then clients will not see anything.
* We send max-lines and max-bytes, this is the hard upper limit.
* A multiline can still be limited more tight if:
  * +f with 't' or 'm' restricts to fewer lines,
    eg +f [5t]:15, which means max 5 lines per 15 seconds,
    means the max accepted multiline is 5 for that channel.
  * +F works the same, except that default +F normal does not
    have a 't' at the moment and 'm' is very high (50) so
    practically not limited by default.
  * There will be a future +f flood subtype for some more control

TODO: we will send CAP NEW on unknown-users <-> known-users to
      indicate the new max-lines value if you transition security groups

TODO: chat history does not yet include multiline batches.

include/struct.h

@@ -1457,7 +1457,10 @@ typedef enum FloodOption {
 	FLD_CONVERSATIONS	= 5,	/**< max-concurrent-conversations */
 	FLD_LAG_PENALTY		= 6,	/**< lag-penalty / lag-penalty-bytes */
 	FLD_VHOST		= 7,	/**< vhost-flood */
+	FLD_MULTILINE		= 8,	/**< multiline max-lines / max-bytes */
 } FloodOption;
+#define MULTILINE_MAX_CONFIGURABLE_LINES	200	/**< Maximum configurable max-lines for multiline */
+#define MULTILINE_MAX_CONFIGURABLE_BYTES	131072	/**< Maximum configurable max-bytes for multiline (128KB) */
 #define MAXFLOODOPTIONS 10
 
 typedef struct TrafficStats TrafficStats;
@@ -2662,6 +2665,9 @@ struct ConfigItem_badword {
 #define SKIP_CTCP	0x8
 #define CHECK_INVISIBLE	0x10
 
+/* Flags for 'flags' in 'can_send_to_user' (and future: can_send_to_channel) */
+#define CAN_SEND_SKIP_SPAMFILTER	0x1
+
 typedef struct GeoIPResult GeoIPResult;
 struct GeoIPResult {
 	char *country_code;