imap: Added custom request perform and response handler functions

Added imap_custom(), which initiates the custom command processing,
and an associated response handler imap_state_custom_resp(), which
handles any responses by sending them to the client as body data.

All untagged responses with the same name as the first word of the
custom request string are accepted, with the exception of SELECT and
EXAMINE which have responses that cannot be easily identified. An
extra check has been provided for them so that any untagged responses
are accepted for them.