Update dovecot2 to 2.2.15, and add an LZ4 option that uses liblz4.

ChangeLog:

Some small fixes and changes to v2.2.14. This release is mainly in the hope
that it could still make it into the next Debian stable instead of v2.2.14 -
mainly because of a couple of new assert crashes that started happening in
v2.2.14 and should be fixed now.

	* Plugins can now print a banner comment in doveconf output
	  (typically the plugin version)
	* Replication plugin now triggers low (instead of high) priority for
	  mail copying operations.
	* IMAP/POP3/ManageSieve proxy: If destination server can't be
	  connected to, retry connecting once per second up to the value of
	  proxy_timeout. This allows quick restarts/upgrades on the backend
	  server without returning login failures.
	* Internal passdb lookups (e.g. done by lmtp/doveadm proxy) wasn't
	  returning failure in some situations where it should have (e.g.
	  allow_nets mismatch)
	* LMTP uses mail_log_prefix now for logging mail deliveries instead of
	  a hardcoded prefix. The non-delivery log prefix is still hardcoded
	  though.

	+ passdb allow_nets=local matches lookups that don't contain an IP
	  address (internally done by Dovecot services)
	+ Various debug logging and error logging improvements
	- Various race condition fixes to LAYOUT=index
	- v2.2.14 virtual plugin crashed in some situations


git-svn-id: svn+ssh://svn.freebsd.org/ports/head@371494 35697150-7ecd-e111-bb59-0022644237b5