[WIP] Patch : Change pg_ident.conf parsing to be the same as pg_hba.conf

Attached is a Patch to change the parsing of pg_ident.conf to make it
similar to pg_hba.conf. 
This is based on Todo Item: 
http://archives.postgresql.org/pgsql-hackers/2011-06/msg02204.php 

Purpose - This will allow to catch syntax errors in pg_ident at the startup
or reload time.
  
  
Changes are described as follows: 
a.      Make the load_ident() functionality same as load_hba, such that it
cleans the previous context, after successful parsing. 
b.      Change the load_ident(), so that parsing can be done during load
time and the parsed lines are saved. 
c.      Change the functionality of parse_ident_usermap() so that parsing is
not done during authentication. 
d.      If load_ident() fails for parsing, it returns false and error is
issued. 
        This point I am not sure, as for pg_hba failure it issues FATAL at
startup. Currently I have kept error handling for load of pg_ident same as
pg_hba  
  
I have done the basic testing and test is still in progress. 
  
Suggestions? 
  
  
With Regards, 
Amit Kapila.