Exposing a new hlua function, available from body or init contexts, that
forcefully disables the sending of email alerts even if the mailers are
defined in haproxy configuration.

This will help for sending email directly from lua.
(prevent legacy email sending from intefering with lua)

check.c is one of the largest file and contains too many things. The
e-mail alerting code is stored there while nothing is in mailers.c.
Let's move this code out. That's only 4% of the code but a good start.
In order to do so, a few tcp-check functions had to be exported.

The file mostly contained struct definitions but there was also a
variable export. Most of the stuff currently lies in checks.h and
should definitely move here!