Convert configuration file parsing to use flex

Previously, repmgr was using a very simple ad-hoc string-based parser,
which had various limitations and allowed configuration files to be
created in a way which could cause confusion and/or unexpected
behaviour.

For example, it accepted strings enclosed in single quotes, but treated
strings enclosed in double quotes literally. A node_name defined thusly:

    node_name="somenode"

would result in the literal value '"somenode"' being used, which could
lead to unobvious errors along the lines of:

    no record found for ""somenode""

The configuration file parser has been adapted from the one used by
PostgreSQL itself, so behaves more-or-less identically (though some
functions such as file inclusion are not supported in repmgr).

This makes configuration parsing more robust and consistent;
additionally, error reporting will be more precise.

Note this does mean that some repmgr.conf items previously accepted
as valid by repmgr will now be rejected; in particular this includes
strings containing spaces which are not enclosed in single quotes.

configfile.h

@@ -317,6 +317,8 @@ 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);
 
+void		parse_configuration_item(t_configuration_options *options, ItemList *error_list, ItemList *warning_list, const char *name, const char *value);
+
 bool		parse_recovery_conf(const char *data_dir, t_recovery_conf *conf);
 
 bool		parse_bool(const char *s,
@@ -342,4 +344,7 @@ void		exit_with_cli_errors(ItemList *error_list, const char *repmgr_command);
 void		print_item_list(ItemList *item_list);
 const char *print_connection_check_type(ConnectionCheckType type);
 
+
+extern bool ProcessConfigFile(FILE *fp, const char *config_file, t_configuration_options *options, ItemList *error_list, ItemList *warning_list);
+
 #endif							/* _REPMGR_CONFIGFILE_H_ */