[master] don't use all of boost namespace

It will unexpectedly hide uint8_t for some systems.

Discussed on jabber.

The error was reported here:
http://git.bind10.isc.org/~tester/builder/BIND10/20120612132001-Solaris10-sparc-GCC/logs/build.out

src/lib/dhcp/tests/pkt4_unittest.cc

@@ -31,7 +31,9 @@ using namespace isc;
 using namespace isc::asiolink;
 using namespace isc::dhcp;
 using namespace isc::util;
-using namespace boost;
+// don't import the entire boost namespace.  It will unexpectedly hide uint8_t
+// for some systems.
+using boost::scoped_ptr;
 
 namespace {