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.

src/init.cpp

@@ -1210,6 +1210,15 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
     LogPrintf("Using config file %s\n", GetConfigFile(gArgs.GetArg("-conf", BITCOIN_CONF_FILENAME)).string());
     LogPrintf("Using at most %i automatic connections (%i file descriptors available)\n", nMaxConnections, nFD);
 
+    // Warn about relative -datadir path.
+    if (gArgs.IsArgSet("-datadir") && !fs::path(gArgs.GetArg("-datadir", "")).is_absolute()) {
+        LogPrintf("Warning: relative datadir option '%s' specified, which will be interpreted relative to the "
+                  "current working directory '%s'. This is fragile, because if bitcoin is started in the future "
+                  "from a different location, it will be unable to locate the current data files. There could "
+                  "also be data loss if bitcoin is started while in a temporary directory.\n",
+            gArgs.GetArg("-datadir", ""), fs::current_path().string());
+    }
+
     InitSignatureCache();
     InitScriptExecutionCache();