Provide opt-out for autofix in overlay functions

[dbaston]

A subset of the overlay functions was recently updated to automatically run ST_MakeValid on invalid input geometries. If the new behavior is to be the default, it should be documented and an opt-out mechanism made available for users concerned with data integrity.

dev list discusison: ​https://lists.osgeo.org/pipermail/postgis-devel/2018-February/026885.html

[dbaston]

Alternative: since an opt-out method would be complex, revert the changes and add documentation showing how a user can easily implement an exception-handling overlay wrappers at the PL/PGSQL level.

[dbaston]

Let's leave blockers with their current release. I have a patch ready to revert the autofix behavior and need to rebase it after merging ​https://github.com/postgis/postgis/pull/264

[robe]

Dan can we commit this? Like by Monday. I want to get out an RC1 soon.

[robe]

As Komzzpa noted, this is the wrong pull request.

Should be - ​https://github.com/postgis/postgis/pull/268

[komzpa]

Another, 180-degree different, approach to it is in ​https://github.com/postgis/postgis/pull/275

Similar to legacy.sql, we can provide a file to import to database to revert to "old" behavior. It installs over extension just fine, and provide strong guarantees on preserving data integrity.

[dbaston]

In 16671:

Remove geometry autofix behavior

References #4040
Closes ​https://github.com/postgis/postgis/pull/268