Handle config file reading at single function

The function handles comments, empty lines, config sections,
CHOOSE, and line continuation, while calling another function
for other lines.

Currently used for config file, sysconf, and WIZKIT.

include/flag.h

@@ -191,6 +191,7 @@ struct instance_flags {
      * behaviour of various NetHack functions and probably warrant
      * a structure of their own elsewhere some day.
      */
+    int parse_config_file_src;  /* hack for parse_config_line() */
     int in_lava_effects;   /* hack for Boots_off() */
     int last_msg;          /* indicator of last message player saw */
     int purge_monsters;    /* # of dead monsters still on fmon list */