Fix copy to INBOX in imap4d.

The case-insensitive "INBOX" refers to "the primary
mailbox for this user", which is stored as "mbox".
Thus a copy to "INBOX" should be interpreted as a
copy to "mbox".

Reference: /n/sources/patch/applied/imap4d-copy-inbox
Date: Sun Jul 29 14:05:57 CES 2012
Signed-off-by: djc@9grid.fr

--- /sys/src/cmd/ip/imap4d/imap4d.c	Sun Jul 29 14:05:46 2012
+++ /sys/src/cmd/ip/imap4d/imap4d.c	Sun Jul 29 14:05:41 2012
@@ -604,6 +604,8 @@
 		Bprint(&bout, "%s NO %s%s bad mailbox\r\n", tg, uid, cmd);
 		return;
 	}
+	if(cistrcmp(mbox, "inbox") == 0)
+		mbox = "mbox";
 	if(!cdExists(mboxDir, mbox)){
 		check();
 		Bprint(&bout, "%s NO [TRYCREATE] %s mailbox does not exist\r\n", tg, cmd);