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_framework.py

@@ -220,18 +220,18 @@ class BitcoinTestFramework():
         for i in range(num_nodes):
             self.nodes.append(TestNode(i, self.options.tmpdir, extra_args[i], rpchost, timewait=timewait, binary=binary[i], stderr=None, mocktime=self.mocktime, coverage_dir=self.options.coveragedir, use_cli=self.options.usecli))
 
-    def start_node(self, i, extra_args=None, stderr=None):
+    def start_node(self, i, *args, **kwargs):
         """Start a bitcoind"""
 
         node = self.nodes[i]
 
-        node.start(extra_args, stderr)
+        node.start(*args, **kwargs)
         node.wait_for_rpc_connection()
 
         if self.options.coveragedir is not None:
             coverage.write_all_rpc_commands(self.options.coveragedir, node.rpc)
 
-    def start_nodes(self, extra_args=None):
+    def start_nodes(self, extra_args=None, *args, **kwargs):
         """Start multiple bitcoinds"""
 
         if extra_args is None:
@@ -239,7 +239,7 @@ class BitcoinTestFramework():
         assert_equal(len(extra_args), self.num_nodes)
         try:
             for i, node in enumerate(self.nodes):
-                node.start(extra_args[i])
+                node.start(extra_args[i], *args, **kwargs)
             for node in self.nodes:
                 node.wait_for_rpc_connection()
         except:
@@ -271,10 +271,10 @@ class BitcoinTestFramework():
         self.stop_node(i)
         self.start_node(i, extra_args)
 
-    def assert_start_raises_init_error(self, i, extra_args=None, expected_msg=None):
+    def assert_start_raises_init_error(self, i, extra_args=None, expected_msg=None, *args, **kwargs):
         with tempfile.SpooledTemporaryFile(max_size=2**16) as log_stderr:
             try:
-                self.start_node(i, extra_args, stderr=log_stderr)
+                self.start_node(i, extra_args, stderr=log_stderr, *args, **kwargs)
                 self.stop_node(i)
             except Exception as e:
                 assert 'bitcoind exited' in str(e)  # node must have shutdown