Compiler: Support clearing of inherited variables in __INIT().

Clearing variables may be significant if the variable has
an earlier initializer in the inherited class. Eg:

  class A { int xyz = 17; }
  class B { inherit A; int xyz = 0; }

Fixes [LysLysKOM 20362124]/Pike mailinglist 13660.
Compiler: Avoid folding some program constants in __INIT().

Program constants that may need their parent must not
be folded be the __INIT optimizer, since doing so will
cause them to lose their parents.
Testsuite: Extended the __INIT() tests somewhat.