Release 0.7.6

Release 0.7.6

* Apply boundary fix for dkim simple canonization
* Fix ping command
* Return nil if header was not found in lua_task
* Fix hang in upstreams revive logic
* Decode entitles when normalizing HTML parts
* Fix logic of finding URLs in HTML parts
* Do not include \0 into length of text when performing conversion to utf8
* Fix raw vs parsed reperesentations
	Raw parts are now:
		- decoded b64/qp, but *NOT* converted to utf-8
	Processed parts are now:
		- converted to UTF-8
		- normalized if needed (e.g. HTML tags are stripped)
* Rework DKIM canonization to line based
* Fix fuzzy hashes addding
* Use more specific hash function for fuzzy
* Fix leaking of iconv descriptors
* Fix PTR resolving in lua resolver
* Rework spf module.
	- Copy data to memory pool as cached record might be destroyed causing
	    freed memory being passed to the protocol output (use after free)
	- Allow SPF_NEUTRAL policy to be handled separately
	- Add R_SPF_NEUTRAL to the default config
* Rework `register_symbols` function
* Allow to disable components of hfilter

Signed-off-by: Vsevolod Stakhov <vsevolod@highsecure.ru>