make selfforward the default

Phil says "we're probably going to have a bad time without it; if we end
up in some permutations where it doesn't work well then we probably
ought to face up and fix it".   Sounds good to me.

packages/mercury/package.py

@@ -42,7 +42,7 @@ class Mercury(CMakePackage):
     variant('cci', default=False, description="Use CCI for network transport")
     variant('bmi', default=False, description="Use BMI for network transport")
     variant('fabric',default=True, description="Use libfabric for net transport")
-    variant('selfforward', default=False, description="Mercury will short-circuit operations by forwarding to itself when possible")
+    variant('selfforward', default=True, description="Mercury will short-circuit operations by forwarding to itself when possible")
 
     # if nothing specified, build good ol' BMI
     depends_on('cci@master', when="+cci", type=("build", "link", "run"))