opkg: vfork external gzip command to uncompress data

Opkg's builtin decompression code is unsuitable to process nested archives as
it uses a single shared state and relies on undefined seek behaviour for pipes.

Rework the extraction logic to use the external gzip command as I/O filter for
decompressing data and remove the builtin inflate code entirely.

This shrinks the final opkg binary by about 4KB and results in less runtime
memory consumption due to efficient use of vfork() and less copy-on-write
operations in the forked child.

Rework by Felix: create a thread that relays data to the gzip process
instead of using a fragile poll loop

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Felix Fietkau <nbd@nbd.name>