MEDIUM: httpcheck/ssl: Base the SNI value on the HTTP host header by default

MEDIUM: httpcheck/ssl: Base the SNI value on the HTTP host header by default

Similarly to the automic SNI selection for regulat SSL traffic, the SNI of
health-checks HTTPS connection is now automatically set by default by using
the host header value. "check-sni-auto" and "no-check-sni-auto" server
settings were added to change this behavior.

Only implicit HTTPS health-checks can take advantage of this feature. In
this case, the host header value from the "option httpchk" directive is used
to extract the SNI. It is disabled if http-check rules are used. So, the SNI
must still be explicitly specified via a "http-check connect" rule.

This patch with should paritally fix the issue #3081.