Add CheckDataDirOption() function

src/util/system.h

@@ -83,6 +83,8 @@ fs::path GetDefaultDataDir();
 // The blocks directory is always net specific.
 const fs::path &GetBlocksDir();
 const fs::path &GetDataDir(bool fNetSpecific = true);
+// Return true if -datadir option points to a valid directory or is not specified.
+bool CheckDataDirOption();
 /** Tests only */
 void ClearDatadirCache();
 fs::path GetConfigFile(const std::string& confPath);