Create the pg_ident_rule defined type

This allows us to declare user map as easilly as pg_hba entries.
Set manage_pg_ident_conf to false for backward compatibility

I'd like to see this patch included ASAP -- the desirable default could
be manage_pg_ident_conf => true, but one could already manage this file
manually : we don't want to wipe it.

Please switch the default from false to true at the next major release
and write a line about this in the release notes.
Merge pull request #452 from txaj/support_pg_ident

Create the pg_ident_rule defined type