checknupdate: make it more reliable

cdn seems to update slower these days and checksum is likely to fail due
to no file when feed update observed

checknupdate: add support for first patch

checknupdate: make it alpine-native

checknupdate: fix when branch is still mainline

match patchlevel dot as well to avoid matching mainline

checknupdate: use json data

checknupdate: fix pkgver parsing

add auto update script