imap: Introduced the mailbox variable

Added the mailbox variable to the per-connection structure in
preparation for checking for an already selected mailbox.
imap: Added skipping of SELECT command if already in the same mailbox

Added storage and checking of the last mailbox userd to prevent
unnecessary switching.
imap: Added processing of more than one response when sent in same packet

Added a loop to imap_statemach_act() in which Curl_pp_readresp() is
called until the cache is drained. Without this multiple responses
received in a single packet could result in a hang or delay.