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/pg_basebackup/streamutil.c

@@ -32,11 +32,11 @@ static char *dbpassword = NULL;
 PGconn	   *conn = NULL;
 
 /*
- * strdup() and malloc() replacements that prints an error and exits
+ * strdup() and malloc() replacements that print an error and exit
  * if something goes wrong. Can never return NULL.
  */
 char *
-xstrdup(const char *s)
+pg_strdup(const char *s)
 {
 	char	   *result;
 
@@ -50,7 +50,7 @@ xstrdup(const char *s)
 }
 
 void *
-xmalloc0(int size)
+pg_malloc0(size_t size)
 {
 	void	   *result;
 
@@ -89,8 +89,8 @@ GetConnection(void)
 	if (dbport)
 		argcount++;
 
-	keywords = xmalloc0((argcount + 1) * sizeof(*keywords));
-	values = xmalloc0((argcount + 1) * sizeof(*values));
+	keywords = pg_malloc0((argcount + 1) * sizeof(*keywords));
+	values = pg_malloc0((argcount + 1) * sizeof(*values));
 
 	keywords[0] = "dbname";
 	values[0] = "replication";