Change usage of extent to st_extent (and do the same for other postgis specific function)

It seems that Postgis 1.4 dropped old functions like "extent" and replaced them with variants such as "st_extent". The ST_* functions are supported since PostGIS 1.2, which is less than 2 years old, so I guess a negotiation dance is required in order to support "not so old" servers.
Locate all the functions that need to be switched and setup a simple version toggle to use the appropriate function names depending on the PostGIS version used.