Add "CAP chghost" support. Internal recode of userhost changes.

Fix force-rejoin not working if doing SVSMODE -x/+x (Koragg, #5015).

Note to module coders:
Please use the following procedure in case of an user/host change:
* userhost_save_current(acptr);
* << change username or hostname here (or both) >>
* userhost_changed(acptr);
This function will take care of notifying other clients about
the userhost change, such as doing PART+JOIN+MODE if force-rejoin
is enabled, and sending :xx CHGHOST user host messages to
"CAP chghost" capable clients.

Also, small note to everyone:
If force-rejoin is enabled we will not send the PART+JOIN+MODE to
"CAP chghost" capable clients. Doing so is just a hack to notify
people of a userhost change. "CAP chghost" users can thus benefit
from the reduced noise in this respect.

src/modules/m_sethost.c

@@ -128,6 +128,8 @@ CMD_FUNC(m_sethost)
 	}
 
 	{
+		userhost_save_current(sptr);
+
 		switch (UHOST_ALLOWED)
 		{
 			case UHALLOW_NEVER:
@@ -147,7 +149,6 @@ CMD_FUNC(m_sethost)
 				}
 				break;
 			case UHALLOW_REJOIN:
-				rejoin_leave(sptr);
 				/* join sent later when the host has been changed */
 				break;
 		}
@@ -165,8 +166,7 @@ CMD_FUNC(m_sethost)
 		/* spread it out */
 		sendto_server(cptr, 0, 0, ":%s SETHOST %s", sptr->name, parv[1]);
 
-		if (UHOST_ALLOWED == UHALLOW_REJOIN)
-			rejoin_joinandmode(sptr);
+		userhost_changed(sptr);
 	}
 
 	if (MyConnect(sptr))