modules-load: implement parallel module loading

modules-load: implement parallel module loading

Load modules in parallel using a pool of worker threads. The number of
threads is equal to the number of CPUs, with a maximum of 16 (to avoid
too many threads being started during boot on systems with many an high
core count, since the number of modules loaded on boot is usually on
the small side).

The number of threads can optionally be specified manually using the
SYSTEMD_MODULES_LOAD_NUM_THREADS environment variable; in this case,
no limit is enforced. If SYSTEMD_MODULES_LOAD_NUM_THREADS is set to 0,
probing happens sequentially.

Co-authored-by: Eric Curtin <ecurtin@redhat.com>