Don't allow relative -walletdir paths

Also warn if bitcoind is configured to use a relative -datadir path.

Specifying paths relative to the current working directory in a daemon process
can be dangerous, because files can fail to be located even if the
configuration doesn't change, but the daemon is started up differently.

Specifying a relative -datadir now adds a warning to the debug log. It would
not be backwards-compatible to forbid relative -datadir paths entirely, and it
could also be also inconvenient for command line testing.

Specifying a relative -walletdir now results in a startup error. But since the
-walletdir option is new in 0.16.0, there should be no compatibility issues.
Another reason not to use working directory paths for -walletdir specifically
is that the default -walletdir is a "wallets" subdirectory inside the datadir,
so it could be surprising that setting -walletdir manually would choose a
directory rooted in a completely different location.

test/functional/test_framework/test_node.py

@@ -81,13 +81,13 @@ class TestNode():
             assert self.rpc_connected and self.rpc is not None, "Error: no RPC connection"
             return getattr(self.rpc, name)
 
-    def start(self, extra_args=None, stderr=None):
+    def start(self, extra_args=None, stderr=None, *args, **kwargs):
         """Start the node."""
         if extra_args is None:
             extra_args = self.extra_args
         if stderr is None:
             stderr = self.stderr
-        self.process = subprocess.Popen(self.args + extra_args, stderr=stderr)
+        self.process = subprocess.Popen(self.args + extra_args, stderr=stderr, *args, **kwargs)
         self.running = True
         self.log.debug("bitcoind started, waiting for RPC to come up")