Which signal to use for CANCEL from postmaster to backend?

Hmm.  I find that SIGUSR1 and SIGUSR2 are both already in use for
communication between backends.  We can't really commandeer SIGURG,
either, because it's apparently the same as SIGUSR1 on SCO
(see src/include/port/sco.h ... so OOB wouldn't work there anyway!).

All three of SIGINT, SIGHUP, SIGTERM currently do the same thing in a
backend, so it looks like our best choice is to redefine one of those
as the cancel request signal.  Any preference?

            regards, tom lane