imap: Add client.event and use it as mail_*user's parent event

Added imap-hibernate process for gathering IDLEing imap processes. imap_hibernate_timeout setting controls how quickly the connection is moved from imap process to imap-hibernate process. Some IMAP extensions like NOTIFY, SEARCH=CONTEXT and COMPRESS aren't supported yet. There's also a new X-STATE command, which can be used to export the current IMAP connection state to a string and later on imported to get back to the original state (a quick resync feature for IMAP clients). However, this command is disabled for now due to the current code being unoptimized for untrusted input.

Avoid using (void)s by adding ATTR_NOWARN_UNUSED_RESULT attributes and other ways.

imap: Added imap_client_created_hook_set() to update the hook.

imap: Added support for verbose_proctitle=yes --HG-- branch : HEAD

imap, pop3: Moved imap/pop3_client_workarounds setting parsing to config checking. --HG-- branch : HEAD

lib-master has now a global master_service variable that all binaries use. There should always be only one of them anyway. --HG-- branch : HEAD

Renamed headers to prevent collision if they were flattened on an install. --HG-- branch : HEAD