Shell: Implement a very basic exec builtin

Other shells also support a number of other options with exec and
some have special behaviour when calling exec with no arguments except
redirections.

This PR only supports the basic case of replacing the Shell process
(or LibShell host process) with the provided command.

Shell/Builtin.cpp

@@ -256,6 +256,20 @@ int Shell::builtin_dirs(int argc, const char** argv)
     return 0;
 }
 
+int Shell::builtin_exec(int argc, const char** argv)
+{
+    if (argc < 2) {
+        fprintf(stderr, "Shell: No command given to exec\n");
+        return 1;
+    }
+
+    Vector<const char*> argv_vector;
+    argv_vector.append(argv + 1, argc - 1);
+    argv_vector.append(nullptr);
+
+    execute_process(move(argv_vector));
+}
+
 int Shell::builtin_exit(int argc, const char** argv)
 {
     int exit_code = 0;