imap: Introduced a custom IMAP structure for per-request data

Created a new IMAP structure and changed the type of the imap proto
variable in connectdata from FTP* to the new IMAP*.

Moved the mailbox variable from the per-connection struct imap_conn to
the new per-request struct and fixed references accordingly.

lib/urldata.h

@@ -1292,7 +1292,7 @@ struct UrlState {
     void *telnet;        /* private for telnet.c-eyes only */
     void *generic;
     struct SSHPROTO *ssh;
-    struct FTP *imap;
+    struct IMAP *imap;
     struct FTP *pop3;
     struct FTP *smtp;
   } proto;