device: combine debug and info log levels into 'verbose'

There are very few cases, if any, in which a user only wants one of
these levels, so combine it into a single level.

While we're at it, reduce indirection on the loggers by using an empty
function rather than a nil function pointer. It's not like we have
retpolines anyway, and we were always calling through a function with a
branch prior, so this seems like a net gain.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/device_test.go

@@ -148,7 +148,7 @@ NextAttempt:
 			} else {
 				p.ip = net.ParseIP("1.0.0.2")
 			}
-			level := LogLevelDebug
+			level := LogLevelVerbose
 			if _, ok := tb.(*testing.B); ok && !testing.Verbose() {
 				level = LogLevelError
 			}