Fixed using of internal NJS headers in nginx modules.

Fixed using of internal NJS headers in nginx modules.

Public API is rectified to make it easier to work with the private
structure njs_value_t from the outside:
    1) njs_vm_retval() is split into njs_vm_retval() which now returns
    the njs_value_t * as a return value and njs_vm_value_to_ext_string()
    which stringifies an njs_value_t * passed as an argument.

    2) njs_value_*_set() methods are added.
    3) Similar public methods are grouped together.