MEDIUM: ssl/crtlist: "crt" keyword in frontend

MEDIUM: ssl/crtlist: "crt" keyword in frontend

This patch implements the "crt" keywords in frontend, declaring an
implicit crt-list named after the frontend.

The patch is split in two steps:

The first step is the crt keyword parser, which parses crt lines and
fill a "cfg_crt_node" struct containing a ssl_bind_conf and a
ckch_conf which are put in a list to be used later.

After parsing the frontend section, as a 2nd step, a
post_section_parser is called, it will create a crt-list named after
the frontend and will fill it with certificates from the list of
cfg_crt_node. Once created this crt-list will be loaded in every "ssl"
bind lines that didn't declare any crt or crt-list.

Example:

    listen https
       bind :443 ssl
       crt foobar.pem
       crt test1.net.crt key test1.net.key

Implements part of #2854