postgis_extension_AddToSearchPath should take input as text instead of varchar

[robe]

This is a security change.

It is possible for a user to create a function postgis_extension_AddToSearchPath(text) in the same schema as the

postgis_extension_AddToSearchPath(varchar) we defined.

This could allow a rogue user to have their version of function run during extension create/updates instead of the one we ship.