Mail: IMAP pipelining support. The change is mostly the same as the SMTP one (04e43d03e153 and 3f5d0af4e40a), and ensures that nginx is able to properly handle or reject multiple IMAP commands. The s->cmd field is not really used and set for consistency. Non-synchronizing literals handling in invalid/unknown commands is limited, so when a non-synchronizing literal is detected at the end of a discarded line, the connection is closed.