Allow negative values in configuration parameters, where appropriate.

Make the code match the documentation.

As pointed out by GitHub user phyber (#142).

Also various other minor improvements to error reporting during
config file parsing.

config.h

@@ -106,6 +106,7 @@ char	   *trim(char *s);
 void		error_list_append(ErrorList *error_list, char *error_message);
 int			repmgr_atoi(const char *s,
 						const char *config_item,
-						ErrorList *error_list);
+						ErrorList *error_list,
+						bool allow_negative);
 
 #endif