Stop using the non-reentrant version of GEOS C-API functions

[strk]

Everytime I add something new to GEOS question myself about adding or not the non-reentrant signature, and everytime I tell to myself: PostGIS ain't ready for that…

Guess it's about time to get ready! Being there GEOS calls in liblwgeom, this might mean having a "liblwgeom" context too, which btw is also useful for other things (reentrancy of error messages!).