Refactor configuration file reload handling

Rather than parse the configuration file into a new structure and
copy changed values from that into the main structure, we'll copy
the existing structure before parsing the changed configuration
file directly into the nmain structure, and revert using the copy
if any issues are encountered.

This is necessary as preparation for further reworking of the
configuration file structure handling. It also makes the reload
idempotent.

While we're at it, make some general improvements to the reload
handling, particularly:

 - improve logging to show "before" and "after" values
 - collate change notifications and only display if no errors
   were found
 - remove unnecessary double-logging of errors
 - various bugfixes

configfile.h

@@ -320,7 +320,7 @@ void		set_progname(const char *argv0);
 const char *progname(void);
 
 void		load_config(const char *config_file, bool verbose, bool terse, t_configuration_options *options, char *argv0);
-bool		reload_config(t_configuration_options *orig_options, t_server_type server_type);
+bool		reload_config(t_server_type server_type);
 
 void		parse_configuration_item(t_configuration_options *options, ItemList *error_list, ItemList *warning_list, const char *name, const char *value);
 
@@ -342,7 +342,7 @@ bool parse_pg_basebackup_options(const char *pg_basebackup_options,
 
 int parse_output_to_argv(const char *string, char ***argv_array);
 void free_parsed_argv(char ***argv_array);
-
+const char *format_failover_mode(failover_mode_opt failover);
 
 /* called by repmgr-client and repmgrd */
 void		exit_with_cli_errors(ItemList *error_list, const char *repmgr_command);