Use task:has_header() to check if header is exists.

These rules are no longer relevant.

 - Support multiple DKIM results
 - Insert DKIM trace symbols
 - Always disable callbacks if we got a header
 - Make the plugin default-disabled
 - Disable callbacks instead of virtual symbols

 - Move neural functions to library
 - Parameterise spawn_train
 - neural plugin: Fix store_pool_only when autotrain is true
 - neural plugin: Use cache_set instead of mempool
 - Add test

Match more variations for undisclosed-recipients, but don't match when
it is used as a lame real name.

These versions are rarely used by real uses nowadays, but can be found
in spam.

FAKE_REPLY_C rule no longer works because MUA it used are no longer used
(and ones which are used changed headers they add).  In theory one can
test all popular modern MUA and see which add only References, which
only In-Reply-To and which add both headers.  But it will be a lot of
work to maintain such rule up to date.  It still has a small number of
hits, but mostly because some spammers use old X-Mailer headers.  This
should be addressed separately.

While here re-create FAKE_REPLY as a regexp module rule - this way it
should be more efficient.

Mail from AOL not longer have X-AOL-Global-Disposition header and this
rule no longer works.  AOL messages now have X-YMail-OSG header (like
yahoo.com), but it is encrypted/obfuscated and cannot be used here.

Pointed by: @citrin

Value for an option is the number of full hours the date of the message
is in the past or future.

The symbol indicates that a date in Date: header has invalid format and
cannot be parsed by Rspamd.

Combine Date header checks into a single callback. No functional
changes.

We should treat invalid selector as a negative result of the check.

Regression was introduced in 2367506006