MINOR: version: make the version strings variables, not constants

MINOR: version: make the version strings variables, not constants

It currently is not possible to figure the exact haproxy version from a
core file for the sole reason that the version is stored into a const
string and as such ends up in the .text section that is not part of a
core file. By turning them into variables we move them to the data
section and they appear in core files. In order to help finding them,
we just prepend an extra variable in front of them and we're able to
immediately spot the version strings from a core file:

  $ strings core | fgrep -A2 'HAProxy version'
  HAProxy version follows
  2.1-dev2-e0f48a-88
  2019/10/15

(These are haproxy_version and haproxy_date respectively). This may be
backported to 2.0 since this part is not support to impact anything but
the developer's time spent debugging.