libopkg: use xsystem() to spawn opkg-key

libopkg: use xsystem() to spawn opkg-key

Instead of the custom fork()/exec() implementation, use the existing
xsystem() helper function which provides a number of benefits:

 - It readily provides error reporting in case the execution fails
 - It has a simpler api
 - It uses vfork() internally which avoids the need to copy pages

This likely fixes https://bugs.openwrt.org/index.php?do=details&task_id=2734.

Fixes: 71f02a3 ("libopkg: add support for signature checking through usign")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>