Create rt_legacy.c, postgis_legacy.c

[robe]

I plan to create two c function files to store legacy functions and each function will just throw an error when called instructing the user to upgrade their install.

This was Darafei's idea and I think a good one even if we don't go into the "Oh my god we are changing so much" that gave pramsey the heebeegeebees in this pull request

​https://github.com/postgis/postgis/pull/302

For starters I will put the pg_abs and the RT_AsBinary thingy functions in herethat were taken out in 2.5.

These I had already changed in 2.5 to stub with errors so symlink still works.

That will allow coming from 2.2 -> 3.0 by symlinking when you do a pg_upgrade.

Next I will find the function we took out in 2.2 (I forget the name) that broke pg_upgrading between 2.1 and 2.2

I think we might have some breakers between 2.0 and 2.1 as well which I'll try to dig up.

I'm hoping all those folks stuck in 2.0 and 2.1 because their database is too big not to do a dump restore can be encouraged to move up to 3.0 if pg_upgrade from 2.0 -> 3.0 is relatively painless