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 generator's
output to a temporary file and checks that the generator succeeded and
wrote at least one AS. Only then does it compress the list and move it
into place. Otherwise it leaves the file alone and exits non-zero.

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: