I would suggest to check for available disk space before running upgrade in
pg_upgradecluster. May be adding some "Are you sure [Y/n]:" interaction in
this case, or just refuse to do upgrade without needed free space. And adding
some "-f" option for overriding that.

This is especially important since now `pg_upgradecluster` is started from
package upgrade script. User most probably will not think carefully about free
space for upgrading at that time. We should inform him explicitly.

Since I both perl developer and deal with PostgreSQL core code on regular
basis I can provide patches for these functionality, if Debian PostgreSQL
Maintainers approve the idea.