SIGQUIT) will be recognized and processed while waiting for input,
rather than only after something has been typed.  Also make SIGQUIT
do the same thing as SIGTERM in single-user mode, ie, do a normal
shutdown and exit.  Since it's relatively easy to provoke SIGQUIT
from the keyboard, people may try that instead of control-D, and we'd
rather this leads to orderly shutdown.  Per report from Leon Mergen
and subsequent discussion.