postrr.sql: Added PostRR_update() functions.

These functions may be used to insert or update a value in an archive. The
functions expect a timestamp and double precision value as arguments to
describe the new values. The postrr.rrarchives will then be used to look up
the actual place where to store the values (there may be multiple ones).

There are currently two overloaded versions of this function. One accepts the
name of an archive and the other expects the definition of an archive as
arguments. The latter is meant to be used for internal purposes while the
other is meant to be used by the user. This is not enforced in any way, though
(and I don't see any reason to do so).