MAJOR: init: automatically set maxconn and/or maxsslconn when possible

MAJOR: init: automatically set maxconn and/or maxsslconn when possible

If a memory size limit is enforced using "-n" on the command line and
one or both of maxconn / maxsslconn are not set, instead of using the
build-time values, haproxy now computes the number of sessions that can
be allocated depending on a number of parameters among which :

  - global.maxconn (if set)
  - global.maxsslconn (if set)
  - maxzlibmem
  - tune.ssl.cachesize
  - presence of SSL in at least one frontend (bind lines)
  - presence of SSL in at least one backend (server lines)
  - tune.bufsize
  - tune.cookie_len

The purpose is to ensure that not haproxy will not run out of memory
when maxing out all parameters. If neither maxconn nor maxsslconn are
used, it will consider that 100% of the sessions involve SSL on sides
where it's supported. That means that it will typically optimize maxconn
for SSL offloading or SSL bridging on all connections. This generally
means that the simple act of enabling SSL in a frontend or in a backend
will significantly reduce the global maxconn but in exchange of that, it
will guarantee that it will not fail.

All metrics may be enforced using #defines to accomodate variations in
SSL libraries or various allocation sizes.