random(4): Poll for signals during large reads

Occasionally poll for signals during large reads of the /dev/u?random
devices. This allows cancellation via SIGINT of accidental invocations of
very large reads. (A 2GB /dev/random read, which takes about 10 seconds on
my 2017 AMD Zen processor, can be aborted.)

I believe this behavior was intended since 2014 (r273997), just not fully
implemented.

This is motivated by a potential getrandom(2) interface that may not
explicitly forbid extremely large reads on 64-bit platforms -- even larger
than the 2GB limit imposed on devfs I/O by default. Such reads, if they are
to be allowed, should be cancellable by the user or administrator.

Reviewed by: delphij
Approved by: secteam (delphij)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D14684