Convert GPDB-specific GUCs to the new "enum" type

Several GUCs are simply enumerated strings that are parsed into integer
types behind the scenes. As of 8.4, the GUC system recognizes a new
type, enum, which will do this for us. Move as many as we can to the new
system.

As part of this,
- gp_idf_deduplicate was changed from a char* string to an int, and new
  IDF_DEDUPLICATE_* macros were added for each option
- password_hash_algorithm was changed to an int
- for codegen_optimization_level, "none" is the default now when codegen
  is not enabled during compilation (instead of the empty string).

A couple of GUCs that *could* be represented as enums
(optimizer_minidump, gp_workfile_compress_algorithm) have been
purposefully kept with the prior system because they require the GUC
variable to be something other than an integer anyway.
Signed-off-by: NJacob Champion <pchampion@pivotal.io>

src/backend/cdb/cdbgroup.c

@@ -5144,9 +5144,9 @@ choose_deduplicate(PlannerInfo *root, List *sortExprs,
 		*numGroups = num_distinct;
 
 	/* we need some calculates above even if the flag is off */
-	if (pg_strcasecmp(gp_idf_deduplicate_str, "force") == 0)
+	if (gp_idf_deduplicate == IDF_DEDUPLICATE_FORCE)
 		return true;
-	if (pg_strcasecmp(gp_idf_deduplicate_str, "none") == 0)
+	if (gp_idf_deduplicate == IDF_DEDUPLICATE_NONE)
 		return false;
 
 	return dedup_cost < naive_cost;

src/include/cdb/cdbvars.h

@@ -1065,7 +1065,11 @@ extern void increment_command_count(void);
 /*
  * switch to control inverse distribution function strategy.
  */
-extern char *gp_idf_deduplicate_str;
+extern int gp_idf_deduplicate;
+
+#define IDF_DEDUPLICATE_AUTO	0
+#define IDF_DEDUPLICATE_NONE	1
+#define IDF_DEDUPLICATE_FORCE	2
 
 /* default to RANDOM distribution for CREATE TABLE without DISTRIBUTED BY */
 extern bool gp_create_table_random_default_distribution;

src/include/libpq/password_hash.h

@@ -26,6 +26,6 @@ typedef enum
 	PASSWORD_HASH_SHA_256
 } PasswdHashAlg;
 
-extern PasswdHashAlg password_hash_algorithm;
+extern int password_hash_algorithm;
 
 #endif

src/include/utils/guc.h

@@ -361,7 +361,6 @@ extern char *application_name;
 extern char *Debug_dtm_action_sql_command_tag;
 extern char *Debug_dtm_action_str;
 extern char *Debug_dtm_action_target_str;
-extern char *Debug_dtm_action_protocol_str;
 
 /* Enable check for compatibility of encoding and locale in createdb */
 extern bool gp_encoding_check_locale_compatibility;