Make assorted quality-of-life improvements in gen_node_support.pl.
Fix incorrect reporting of the location of errors (such as bogus
node attributes). Add header comments to the generated files,
containing copyright notices and reminders that they are generated
files, as we do in other file-generating scripts. Arrange to not
leave a clutter of temporary files when the script detects an error.
Discussion: https://postgr.es/m/
3843645.
1657385930@sss.pgh.pa.us