BUG/MEDIUM: init: fix fd_hard_limit default in compute_ideal_maxconn

BUG/MEDIUM: init: fix fd_hard_limit default in compute_ideal_maxconn

This commit fixes 41275a691 ("MEDIUM: init: set default for fd_hard_limit via
DEFAULT_MAXFD").

fd_hard_limit is taken in account implicitly via 'ideal_maxconn' value in
all maxconn adjustements, when global.rlimit_memmax is set:

	MIN(global.maxconn, capped by global.rlimit_memmax, ideal_maxconn);

It also caps provided global.rlimit_nofile, if it couldn't be set as a current
process fd limit (see more details in the main() code).

So, lets set the default value for fd_hard_limit only, when there is no any
other haproxy-specific limit provided, i.e. rlimit_memmax, maxconn,
rlimit_nofile. Otherwise we may break users configs.

Please, note, that in master-worker mode, master does not need the
DEFAULT_MAXFD (1048576) as well, as we explicitly limit its maxconn to 100.

Must be backported in all stable versions until v2.6.0, including v2.6.0,
like the commit above.

(cherry picked from commit 16a5fac4bba1cb2bb6cf686066256aa141515feb)
Signed-off-by: Willy Tarreau <w@1wt.eu>