some symbol tweaks

A few symbol-related modifications:

- fulfill a request from a blind player to allow them to
  specify a unique/recognizable character for all pets and/or
  the player in the config file for use when using a screen
  reader (S_player_override, S_pet_override). Requires sysconf
  setting ACCESSIBILITY to be set to have an effect, although
  they can still be specified in the config file.

- Config file SYMBOLS entries were not working properly on
  the rogue level. Allow ROGUESYMBOLS as well as SYMBOLS to be
  specified in the config file independently.

- When values are moved into showsyms[], the overriding SYMBOLS
  or ROGUESYMBOLS entry from the config file is used if there is
  one; if there is no overriding value for a particular symbol,
  the loaded symset value is used; if there is no symset entry
  loaded for the symbol then a default symbol is used.

src/sys.c

@@ -80,6 +80,7 @@ sys_early_init()
     sysopt.check_plname = 0;
     sysopt.seduce = 1; /* if it's compiled in, default to on */
     sysopt_seduce_set(sysopt.seduce);
+    sysopt.accessibility = 0;
     return;
 }