libbb: use external gzip command as I/O layer

libbb: use external gzip command as I/O layer

Remove the builtin inflate code and rely on the external gzip command instead
to inflate data streams.

This change is needed to properly support vfork(). We cannot use the builtin
code because it uses a single global state which will get clobbered when
recursively inflating nested archives.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>