Use CREATE OR REPLACE for aggregates when possible

[strk]

Thanks to Andrew Gierth (RhodiumToad on IRC) PostgreSQL 12 will have CREATE OR REPLACE AGGREGATE. We could start using it from the upgrade script:

​https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=01bde4fa4c24f4eea0a634d8fcad0b376efda6b1

[strk]

Example of the problem we'll be able to fix with PostgreSQL 12 if we implement this: #4386

[strk]

Another example we'd be able to fix: #4334

[Algunenano]

WIP in ​https://github.com/postgis/postgis/pull/414

[algunenano]

In 17488:

Use CREATE OR REPLACE AGGREGATE for PG12+

References #4352

[algunenano]

In 17489:

Perl tidy over postgis_proc_upgrade.pl

To match PostgreSQL convenctions I've used
perltidy -b -bl -nsfs -naws -l=100 -ole=unix postgis_proc_upgrade.pl

Closes #4352