better default support for parallel compression

[Mike Frysinger]

with the rise of commodity multicore computing, tar feels a bit antiquated in
that it still defaults to single (de)compression.  it feels like the defaults
could & should be more intelligent.  has any thought been given to supporting
parallel (de)compression by default ?

i grok that i could just run configure myself and point the various --with-X
knobs to a local parallel program.  but that doesn't really help anyone other
than me, and it doesn't play well when moving between systems that might have
a different set of compression programs available.  i also grok that i could
just pass -I in my own personal invocation, but that's clunky, and doesn't
help with tools that run tar for me, or the users i support.

tar has a minor toe in the water here already:
src/buffer.c:
static struct zip_program const zip_program[] = {
  { ct_bzip2,    BZIP2_PROGRAM,    "-j" },
  { ct_bzip2,    "lbzip2",         "-j" },
but that will rarely, if ever really, hit the lbzip2 program for people.

i also get that there's probably reluctance to make changes in such core
behavior (going from 1 core to all the cores), but it's really hard to
square this away as a reasonable default.

along those lines, communicating preferences as to how many cores the user
wants to utilize will be fun.  but i don't think it's intractable.

i didn't find anything in the archives, so if there's an existing thread on
the topic, feel free to highlight it
-mike

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