MAJOR: config: remove parsing of the global "nbproc" directive

MAJOR: config: remove parsing of the global "nbproc" directive

This one was deprecated in 2.3 and marked for removal in 2.5. It suffers
too many limitations compared to threads, and prevents some improvements
from being engaged. Instead of a bypassable startup error, there is now
a hard error.

The parsing code was removed, and very few obvious cases were as well.
The code is deeply rooted at certain places (e.g. "for" loops iterating
from 0 to nbproc) so it will not be that trivial to remove everywhere.
The "bind" and "bind-process" parsers will have to be adjusted, though
maybe not completely changed if we later want to support thread groups
for large NUMA machines. Some stats socket restrictions were removed,
and the doc was updated according to what was done. A few places in the
doc still refer to nbproc and will have to be revisited. The master-worker
code also refers to the process number to distinguish between master and
workers and will have to be carefully adjusted. The MAX_PROCS macro was
reset to 1, this will at least reduce the size of some remaining arrays.

Two regtests were dependieng on this directive, one with an explicit
"nbproc 1" and another one testing the master's CLI using nbproc 4.
Both were adapted.