Use dynamic context for all backend databases

Rather than defining a static set of database functions use a well
known initialisation function for each database backend that then
returns a database context structure. This structure contains function
pointers for all of the functions previously held in dbstructs as well
as a private instance context pointer.

For the moment this doesn't provide any change to behaviour, but it
provides the initial preparation for allowing multiple database instance
(whether of the same or differing types) to be used at the same time.