__builtin: Changed to directory node.

This change is to allow for a suitable namespace for
holding Pike code that is used by C-modules.
Compiler: low_inherit() now updates parent flags.

Moved update of parent flags (PROGRAM_{NEEDS,USES}_PARENT) from
compiler_do_inherit() to low_inherit(), to help out C-modules
that inherit programs needing parent pointers, since they
typically use low_inherit() directly.
precompile: Support inherit of Pike code.

Added syntax:

  INHERIT "__builtin.foo"
    attributes;

to the precompiler to allow for simple inherit of
code written in Pike. To enable the feature, the
API level must be set to 5 or higher.