Revert "opkg: use vfork on gz_open by default (FS#120)"

This reverts commit 763f5d78.

Currently the vfork() code path in opkg is broken and relies on unsupported
ftello() / fseeko() operations on pipes - we need to restructure the code
before we can reconsider this approach.

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