Add optional config file sections, selected with CHOOSE

Allows the user to define arbitrarily named optional sections
in the config file, and select which of those sections are used.

For example:

  OPTIONS=color
  CHOOSE=char A,char B

  [char A]
  OPTIONS=role:arc,race:dwa,align:law,gender:fem

  [char B]
  OPTIONS=role:wiz,race:elf,align:cha,gender:mal

doc/fixes36.1

@@ -627,6 +627,7 @@ blinded hero or monster who breathes vapor from broken potion of healing,
 	potion: full == always; extra == if not-cursed; plain == if blessed)
 sysconf definition BONES_POOLS to allow more bones files per level
 blessed scroll of fire lets you choose explosion location like stinking cloud
+optional sections in the config file, selected with CHOOSE
 
 
 Platform- and/or Interface-Specific New Features