Upgrading Dovecot CE from 2.4 to 2.4.x

Dovecot CE

Added MALLOC_MMAP_THRESHOLD_=131072 to import_environment when building with glibc. Note that dovecot_config_version value does not affect change - it's always added.

These changes don't take effect until dovecot_config_version is changed from 2.4.0 to 2.4.1.

Setting	Old Default Value	New Default Value
`mail_cache_fields`	flags	flags hdr.date hdr.subject hdr.from hdr.sender hdr.reply-to hdr.to hdr.cc hdr.bcc hdr.in-reply-to hdr.message-id date.received size.virtual imap.bodystructure mime.parts hdr.references hdr.importance hdr.x-priority hdr.x-open-xchange-share-url pop3.uidl pop3.order
`service/auth/unix_listener/auth-userdb/group`	(empty = root)	`$SET:default_internal_group`
`service/lmtp/restart_request_count`	unlimited	1