main: Add a working directory option.

This is an alternative method for specifying the working directory.
Normally, the user would change the directory before executing dingusppc to set the working directory.
The working directory is the default location for files and sockets used by dingusppc (the bootrom file, symbols file, nvram, pram, serial port sockets, logs, etc.)

main.cpp

@@ -39,6 +39,11 @@ along with this program.  If not, see <https://www.gnu.org/licenses/>.
 #include <iostream>
 #include <CLI11.hpp>
 #include <loguru.hpp>
+#ifdef _WIN32
+#include <direct.h>
+#else
+#include <unistd.h>
+#endif
 
 using namespace std;
 
@@ -64,6 +69,21 @@ static string appDescription = string(
     "\n"
 );
 
+/// Check for an existing directory (returns error message if check fails)
+class WorkingDirectoryValidator : public CLI::detail::ExistingDirectoryValidator {
+  public:
+    WorkingDirectoryValidator() {
+        func_ = [](std::string &filename) {
+            std::string result = CLI::ExistingDirectory.operator()(filename);
+            if (result.empty())
+                _chdir(filename.c_str());
+            return result;
+        };
+    }
+};
+
+const WorkingDirectoryValidator WorkingDirectory;
+
 void run_machine(std::string machine_str, std::string bootrom_path, uint32_t execution_mode, uint32_t profiling_interval_ms);
 
 int main(int argc, char** argv) {
@@ -77,11 +97,14 @@ int main(int argc, char** argv) {
     bool   realtime_enabled = false;
     bool   debugger_enabled = false;
     string bootrom_path("bootrom.bin");
+    string working_directory_path(".");
 
     app.add_flag("-r,--realtime", realtime_enabled,
         "Run the emulator in real-time");
     app.add_flag("-d,--debugger", debugger_enabled,
         "Enter the built-in debugger");
+    app.add_option("-w,--workingdir", working_directory_path, "Specifies working directory")
+        ->check(WorkingDirectory);
     app.add_option("-b,--bootrom", bootrom_path, "Specifies BootROM path")
         ->check(CLI::ExistingFile);
     app.add_flag("--deterministic", is_deterministic,