Add new internal static library libthread.a that provides infrastructure
for eu-* tools to run functions concurrently using pthreads.
threadlib.c manages per-job threads as well as per-job buffers for stdout
output. Output for each job is printed to stdout in the order that the
jobs were added to
Implement concurrent execution of print_debug_* functions during handling
of -w, --debug-dump using libthread.a.
A new `-C, --concurrency=NUM` command line option controls the maximum
number of worker threads used. This value defaults to the number of CPUs.
Job output is buffered and printed in t
