Change the way string relopts are allocated.

Don't try to allocate the default value for a string relopt in the same
palloc chunk as the relopt_string struct. That didn't work too well if you
added a built-in string relopt in the stringRelOpts array, as it's not
possible to have an initializer for a variable length struct in C. This
makes the code slightly simpler too.

While we're at it, move the call to validator function in
add_string_reloption to before the allocation, so that if someone does pass
a bogus default value, we don't leak memory.

src/include/access/reloptions.h

@@ -108,7 +108,7 @@ typedef struct relopt_string
 	int			default_len;
 	bool		default_isnull;
 	validate_string_relopt validate_cb;
-	char		default_val[1]; /* variable length, zero-terminated */
+	char	   *default_val;
 } relopt_string;
 
 /* This is the table datatype for fillRelOptions */