luci-mod-system: file naming improvement for repo key management

luci-mod-system: file naming improvement for repo key management

When downloading a key, try to get the filename of the key. opkg expects it to
match the key fingerprint when verifying .sig files. apk not so.

Try to save the public key file with the name from the provided URL, so it is
not named "key_blah" which won't match its fingerprint.

The alternative would be to call usign with the public key file as input, and
then rename it to its fingerprint. This is only necessary for opkg which is on
the way out.

If the public key file at the remote resource is not named according to its
fingerprint, that's on the repo owner.

Signed-off-by: Paul Donald <[email protected]>