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.

sys/unix/sysconf

@@ -61,6 +61,11 @@ MAXPLAYERS=10
 # incubi to use nymphs' charm behavior rather than their own seduce behavior.
 #SEDUCE=0
 
+# Uncomment the next line to enable some accessibility features such
+# as S_player_override and S_pet_override symbols for screen readers
+# in the user config file.
+#ACCESSIBILITY=1
+
 # Uncomment to disable savefile UID checking.
 #CHECK_SAVE_UID=0