imap: Introduced the mailbox variable

Added the mailbox variable to the per-connection structure in
preparation for checking for an already selected mailbox.

lib/imap.c

@@ -1705,6 +1705,7 @@ static CURLcode imap_disconnect(struct connectdata *conn, bool dead_connection)
   Curl_sasl_cleanup(conn, imapc->authused);
 
   /* Cleanup our connection based variables */
+  Curl_safefree(imapc->mailbox);
   Curl_safefree(imapc->mailbox_uidvalidity);
 
   return CURLE_OK;

lib/imap.h

@@ -76,6 +76,7 @@ struct imap_conn {
   bool tls_supported;         /* StartTLS capability supported by server */
   bool login_disabled;        /* LOGIN command disabled by server */
   bool ir_supported;          /* Initial response supported by server */
+  char *mailbox;              /* The last selected mailbox */
   char *mailbox_uidvalidity;  /* UIDVALIDITY parsed from select response */
 };