Fix extbans in +I not being converted to letter bans to older servers.

This affects servers without NEXTBANS, such as anope 2.0.x series
(anope 2.1.x is not affected as it supports NEXTBANS).

Non-NEXTBANS servers only support letter extbans so we are supposed
to convert ~security-group:known-users to ~G:known-users when sending
to such a server, in unreal_server_compat. And we did this well for
the MODE command for +beI. In SJOIN we did this correctly for +b/+e
but not for +I due to a silly code mistake.

This bug is present since 6.0.0 but wasn't noticed until now.

To be a real problem you need something like:
1. Anope 2.0.x series (or other services without NEXTBANS)
2. A channel with +I extbans
3. KEEPMODES set on that channel

Then what happens is when services boot:
1. UnrealIRCd will sync with anope 2.0.x and incorrectly send
   named bans, which will confuse anope. But nothing strange
   happens yet at this point.
2. Then on next server sync (eg anope restart or unreal restart)
   anope will try to restore these but they end up with weird
   entries like +I *!*@~security-group:known-users
   (note the *!*@ prefix)

And it should be noted that this would also happen in a situation
with UnrealIRCd 5 + UnrealIRCd 6 servers, but UnrealIRCd 5 is
End Of Life anyway.

Reported by BlackBishop and Sadie two days ago. Thanks!

src/modules/unreal_server_compat.c

@@ -242,7 +242,7 @@ int usc_reparse_sjoin(char **msg, char *p, int *length)
 				           log_data_string("ban", s));
 				continue;
 			}
-			if (!strchr("&\"\\", next[1]))
+			if (!strchr("&\"'", next[1]))
 				goto fallback_usc_reparse_sjoin;
 			*next++ = '\0';
 			result = clean_ban_mask(next+1, MODE_ADD, EXBTYPE_BAN, &me, NULL, 1); // some context lost
@@ -259,7 +259,7 @@ int usc_reparse_sjoin(char **msg, char *p, int *length)
 			strlcat(obuf, result, sizeof(obuf)); /* the converted result */
 			strlcat(obuf, " ", sizeof(obuf));
 		} else
-		if (strchr("&\"\\", *s))
+		if (strchr("&\"'", *s))
 		{
 			/* +b / +e / +I */
 			const char *result = clean_ban_mask(s+1, MODE_ADD, EXBTYPE_BAN, &me, NULL, 1); // some context lost