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>

package/system/opkg/patches/280-use-vfork.patch

---- a/libbb/gz_open.c
-+++ b/libbb/gz_open.c
-@@ -29,7 +29,7 @@
- #include <unistd.h>
- #include "libbb.h"
- 
--static int gz_use_vfork;
-+static const int gz_use_vfork = 1;
- 
- FILE *
- gz_open(FILE *compressed_file, int *pid)
-@@ -38,8 +38,6 @@ gz_open(FILE *compressed_file, int *pid)
- 	off_t floc;
- 	int cfile = -1;
- 
--	gz_use_vfork = (getenv("OPKG_USE_VFORK") != NULL);
--
- 	if (gz_use_vfork) {
- 		/* Create a new file descriptor for the input stream
- 		 * (it *must* be associated with a file), and lseek()