run-command.c: remove "max_processes", add "const" to signal() handler

run-command.c: remove "max_processes", add "const" to signal() handler

As with the *_fn members removed in a preceding commit, let's not copy
the "processes" member of the "struct run_process_parallel_opts" over
to the "struct parallel_processes".

In this case we need the number of processes for the kill_children()
function, which will be called from a signal handler. To do that
adjust this code added in c553c72eed6 (run-command: add an
asynchronous parallel child processor, 2015-12-15) so that we use a
dedicated "struct parallel_processes_for_signal" for passing data to
the signal handler, in addition to the "struct parallel_process" it'll
now have access to our "opts" variable.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>