Fix buildworld by adding DPADD= to libdpv [dpv(3)] Makefile
NB: Should also address `make -j' building
Remove "+" from "+=" in assignments to DPADD/LDADD while here.
NB: Also move CFLAGS for style measure.

Reviewed by:	shurd
MFC after:	21 days
X-MFC-to:	stable/10 stable/9
X-MFC-with:	274116 274120 274121
Temporarily _disable_ compilation of dpv(3) and dpv(1).
Will revisit this to find out how to solve the ordering
issue in buildworld (potentially `make -j' specific).

Reviewed by:	shurd
Convert to use libxo.

Obtained from:	Phil Shafer <phil@juniper.net>
Sponsored by:	Juniper Networks, Inc.
Document that wc(1) supports libxo(3).
In alloc_pread() and kern_pread(), print errors only when DEBUG is
defined. An error is not fatal and is supposed to be handled by the
caller.

Obtained from:	Juniper Networks, Inc.