The implementation was copied from rogue4. Using fdopen() is necessary
because the scorefile needs both encread() and encwrite(). For some
reason I have failed to discover, one of them uses FILE *'s and the
other uses file descriptors.
md_shellescape() sets SIGINT and SIGQUIT to be ignored, storing the
previous handlers, and restores them after the shell exits. But it
mixed up the two handlers.
Since the signals were usually handled by the same function, this fix
doesn't have much effect, but anything that makes signal code less
confusing is a good thing.
