Fix single-user mode so that interrupts (particularly SIGTERM and

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.