term: wip: track current working directory

This sets the initial current working directory, as it is when the
terminal is instantiated.

We chdir() to it just before spawning a new terminal.

terminal.h

@@ -316,6 +316,7 @@ struct terminal {
     void *shutdown_data;
 
     char *foot_exe;
+    char *cwd;
 };
 
 struct config;