Standardize naming of malloc/realloc/strdup wrapper functions.

We had a number of variants on the theme of "malloc or die", with the
majority named like "pg_malloc", but by no means all.  Standardize on the
names pg_malloc, pg_malloc0, pg_realloc, pg_strdup.  Get rid of pg_calloc
entirely in favor of using pg_malloc0.

This is an essentially cosmetic change, so no back-patch.  (I did find
a couple of places where psql and pg_dump were using plain malloc or
strdup instead of the pg_ versions, but they don't look significant
enough to bother back-patching.)

src/bin/psql/common.c

@@ -70,26 +70,12 @@ pg_malloc(size_t size)
 }
 
 void *
-pg_malloc_zero(size_t size)
+pg_malloc0(size_t size)
 {
 	void	   *tmp;
 
 	tmp = pg_malloc(size);
-	memset(tmp, 0, size);
-	return tmp;
-}
-
-void *
-pg_calloc(size_t nmemb, size_t size)
-{
-	void	   *tmp;
-
-	tmp = calloc(nmemb, size);
-	if (!tmp)
-	{
-		psql_error("out of memory\n");
-		exit(EXIT_FAILURE);
-	}
+	MemSet(tmp, 0, size);
 	return tmp;
 }