Merge branch 'ab/run-hook-api-cleanup'

Merge branch 'ab/run-hook-api-cleanup'

Move a global variable added as a hack during regression fixes to
its proper place in the API.

* ab/run-hook-api-cleanup:
  run-command.c: remove "max_processes", add "const" to signal() handler
  run-command.c: pass "opts" further down, and use "opts->processes"
  run-command.c: use "opts->processes", not "pp->max_processes"
  run-command.c: don't copy "data" to "struct parallel_processes"
  run-command.c: don't copy "ungroup" to "struct parallel_processes"
  run-command.c: don't copy *_fn to "struct parallel_processes"
  run-command.c: make "struct parallel_processes" const if possible
  run-command API: move *_tr2() users to "run_processes_parallel()"
  run-command API: have run_process_parallel() take an "opts" struct
  run-command.c: use designated init for pp_init(), add "const"
  run-command API: don't fall back on online_cpus()
  run-command API: make "n" parameter a "size_t"
  run-command tests: use "return", not "exit"
  run-command API: have "run_processes_parallel{,_tr2}()" return void
  run-command test helper: use "else if" pattern