Pull up following revision(s) (requested by rmind in ticket #140):

external/bsd/libnv/lib/Makefile: revision 1.3
usr.sbin/npf/npfd/Makefile: revision 1.7
lib/libnpf/Makefile: revision 1.11
lib/libnpf/Makefile: revision 1.12
libexec/identd/Makefile: revision 1.18
usr.sbin/npf/npfctl/Makefile: revision 1.14
lib/npf/mod.mk: revision 1.8
external/bsd/libnv/lib/srcs.mk: revision 1.1

libnpf fix: link the library itself to libnv; libnpf users should not be
expected to assume this dependency.

Move common parts into a makefile fragment usable from elsewhere.
Suggested by mrg.

Include libnv directly into this library

(martin)