sysext: support distribution-independent extensions using ID=_any

A sysext image that merely contains static binaries has no dependency
on the host distribution and should be able to be used anywhere.
Support the special '_any' value for the ID field in the extension to
opt-out of ID and VERSION_ID/SYSEXT_LEVEL matching.

See https://github.com/systemd/systemd/issues/24061

test/test-functions

@@ -712,6 +712,12 @@ EOF
         chmod +x "$initdir/opt/script1.sh"
         echo MARKER=1 >"$initdir/usr/lib/systemd/system/other_file"
         mksquashfs "$initdir" "$oldinitdir/usr/share/app1.raw" -noappend
+
+        export initdir="$TESTDIR/app-nodistro"
+        mkdir -p "$initdir/usr/lib/extension-release.d" "$initdir/usr/lib/systemd/system"
+        ( echo "ID=_any" ) >"$initdir/usr/lib/extension-release.d/extension-release.app-nodistro"
+        echo MARKER=1 >"$initdir/usr/lib/systemd/system/some_file"
+        mksquashfs "$initdir" "$oldinitdir/usr/share/app-nodistro.raw" -noappend
     )
 }