Hello,
attached patch is dumb, and allows to use negative timeout, which
results in a quick (non-waiting) single fcntl() call. Such timeout can
already be passed to zsystem flock, because (Src/Modules/system.c):

                timeout = (time_t)mathevali(optarg);

and time_t is signed on Darwin, a linux box that a person tested and I
guess it's always signed because "time can be negative" – but a
knowledge here would be needed.

Best regards,
Sebastian Gniazdowski