Parse pg_ident.conf when it's loaded, keeping it in memory in parsed format.

Similar changes were done to pg_hba.conf earlier already, this commit makes
pg_ident.conf to behave the same as pg_hba.conf.

This has two user-visible effects. First, if pg_ident.conf contains multiple
errors, the whole file is parsed at postmaster startup time and all the
errors are immediately reported. Before this patch, the file was parsed and
the errors were reported only when someone tries to connect using an
authentication method that uses the file, and the parsing stopped on first
error. Second, if you SIGHUP to reload the config files, and the new
pg_ident.conf file contains an error, the error is logged but the old file
stays in effect.

Also, regular expressions in pg_ident.conf are now compiled only once when
the file is loaded, rather than every time the a user is authenticated. That
should speed up authentication if you have a lot of regexps in the file.

Amit Kapila

src/include/libpq/hba.h

@@ -13,6 +13,7 @@
 
 #include "libpq/pqcomm.h"	/* pgrminclude ignore */	/* needed for NetBSD */
 #include "nodes/pg_list.h"
+#include "regex/regex.h"
 
 
 typedef enum UserAuth
@@ -82,11 +83,21 @@ typedef struct HbaLine
 	int			radiusport;
 } HbaLine;
 
+typedef struct IdentLine
+{
+	int 		linenumber;
+
+	char	   *usermap;
+	char	   *ident_user;
+	char	   *pg_role;
+	regex_t 	re;
+} IdentLine;
+
 /* kluge to avoid including libpq/libpq-be.h here */
 typedef struct Port hbaPort;
 
 extern bool load_hba(void);
-extern void load_ident(void);
+extern bool load_ident(void);
 extern void hba_getauthmethod(hbaPort *port);
 extern int check_usermap(const char *usermap_name,
 			  const char *pg_role, const char *auth_user,