Change HOME to /root on exec

This affected some programs like vim, which put root-owned swap files
into the normal user's swap directory, since $HOME didn't change.

Fixes #11

rdo.c

@@ -40,6 +40,8 @@ void runprog(char** program_argv) {
 	if (setgid(0) < 0)
 		err(1, "Could not setgid");
 
+	putenv("HOME=/root");
+
 	// NOTE: this does not return when no error occurred.
 	execvp(program_argv[0], program_argv);