add semantics to dns_nametree to support bitfields

add semantics to dns_nametree to support bitfields

name trees can now hold either boolean values or bit fields. the
type is selected when the name tree is created.

the behavior of dns_nametree_add() differs slightly beteween the types:
in a boolean tree adding an existing name will return ISC_R_EXISTS,
but in a bitfield tree it simply sets the specified bit in the bitfield
and returns ISC_R_SUCCESS.