Keep the AS list when the asupdate generator fails (closes #173)

make asupdate piped the generator into gzip, so the target saw gzip's
exit status. A failed download replaced pkg/asinfo/asdata.json.gz with
an empty file and the target still exited 0.

The target now runs the new script/asupdate. It writes the output of
cmd/asinfo-gen to a temporary file. Only when the generator succeeds
does it compress the list and move it into place. Otherwise it leaves
the file alone and exits non-zero. The generator now fails when as.csv
holds no AS.

The generator reads as.csv from ipverse/as-metadata, the source's new
name. The list it builds there is unchanged, so no new list is
committed.

Model: opus-5-5

pkg/asinfo/doc.go

@@ -2,7 +2,7 @@
 Package asinfo provides information about Autonomous Systems (AS).
 
 The package embeds a comprehensive database of AS numbers, handles, and descriptions
-sourced from https://github.com/ipverse/asn-info. The data is embedded at compile
+sourced from https://github.com/ipverse/as-metadata. The data is embedded at compile
 time and provides fast, offline lookups.
 
 Basic usage: