BUG/MEDIUM: limits: properly account for global.maxpipes in compute_ideal_maxconn()

BUG/MEDIUM: limits: properly account for global.maxpipes in compute_ideal_maxconn()

Starting a config with maxpipes and no maxconn always ended up in error
because the number of FDs needed for pipes was not deduced from the total
number of FDs when calculating maxconn, and was later found to exceed the
total number of allocatable FD during final checks.

When global.maxpipes is set, it must be used during compute_ideal_maxconn()
so that it's properly deduced.

Without this, just having "maxpipes 500" in a config prevents it from
starting. With the fix, it properly starts with a maxconn adjusted
depending on the number of splice-enabled proxies.

This should be backported, theoretically everywhere, but preferably
progressively. The following config should fail on affected versions
and load with fixed ones:

   global
        maxpipes 500

   frontend srv1
        bind :8001