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/imap.h

@@ -23,6 +23,7 @@
  ***************************************************************************/
 
 #include "pingpong.h"
+#include "ftp.h"
 
 /****************************************************************************
  * IMAP unique setup
@@ -51,11 +52,22 @@ typedef enum {
   IMAP_LAST          /* never used */
 } imapstate;
 
+/* This IMAP struct is used in the SessionHandle. All IMAP data that is
+   connection-oriented must be in imap_conn to properly deal with the fact that
+   perhaps the SessionHandle is changed between the times the connection is
+   used. */
+struct IMAP {
+  curl_off_t *bytecountp;
+  char *user;             /* User name string */
+  char *passwd;           /* Password string */
+  curl_ftptransfer transfer;
+  char *mailbox;          /* Mailbox to select */
+};
+
 /* imap_conn is used for struct connection-oriented data in the connectdata
    struct */
 struct imap_conn {
   struct pingpong pp;
-  char *mailbox;          /* Mailbox to select */
   unsigned int authmechs; /* Accepted authentication mechanisms */
   unsigned int authused;  /* Auth mechanism used for the connection */
   imapstate state;        /* Always use imap.c:state() to change state! */