run-command.c: use designated init for pp_init(), add "const"

Use a designated initializer to initialize those parts of pp_init()
that don't need any conditionals for their initialization, this sets
us on a path to pp_init() itself into mostly a validation and
allocation function.

Since we're doing that we can add "const" to some of the members of
the "struct parallel_processes", which helps to clarify and
self-document this code. E.g. we never alter the "data" pointer we
pass t user callbacks, nor (after the preceding change to stop
invoking online_cpus()) do we change "max_processes", the same goes
for the "ungroup" option.

We can also do away with a call to strbuf_init() in favor of macro
initialization, and to rely on other fields being NULL'd or zero'd.

Making members of a struct "const" rather that the pointer to the
struct itself is usually painful, as e.g. it precludes us from
incrementally setting up the structure. In this case we only set it up
with the assignment in run_process_parallel() and pp_init(), and don't
pass the struct pointer around as "const", so making individual
members "const" is worth the potential hassle for extra safety.

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

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