pgtopo_import: add switch to request pre-dropping an existing topology

[strk]

Like shp2pgsql we could add a -d for "drop" switch that removes an existing topology with the same name as the requested one.

The -d idea comes from shp2pgsql - see ​https://postgis.net/docs/using_postgis_dbmanagement.html#load-data-sql

[strk]

Better to go —drop-topology, as it's a big gun we're providing to shoot your foot

[strk]

See also #5860 which could make it easier to tell when —drop-topology would be useful

[Sandro Santilli <strk@…>]

In 1237c6b/git:

Add —drop-topology switch to pgtopo_import

Closes #5861

[Sandro Santilli <strk@…>]

In f0335cae/git:

Document new pgtopo_import switch

References #5861