BUG/MEDIUM: config: properly compute the default number of processes for a proxy

BUG/MEDIUM: config: properly compute the default number of processes for a proxy

Chad Lavoie reported an interesting regression caused by the latest
updates to automatically detect the processes a peers section runs on.
It turns out that if a config has neither nbproc nor a bind-process
statement and depending on the frontend->backend chaining, it is possible
to evade all bind_proc propagations, resulting in assigning only ~0UL (all
processes, which is 32 or 64) without ever restricting it to nbproc. It
was not visible in backends until they started to reference peers sections
which saw themselves with 64 processes at once.

This patch addresses this by replacing all those ~0UL with nbits(nbproc).
That way all "bind-process" settings *default* to the number of processes
defined in nbproc instead of 32 or 64.

This fix could possibly be backported into 1.5, though there is no indication
that this bug could have any effect there.