Simplify kbrequest management

 - the ioctl never works on /dev/console, so trying to use it on stdin
is useless.
 - /dev/tty0 always exists in a devtmpfs, so the late test is okay.
It works on serial consoles too. If no devtmpfs, creating a correct
/dev/tty0 is the responsibility of the integrator.
 - Also added SIGINT blocking before disablecad, to avoid potential
early panic. For it to work on kernels without signalfd, it requires
the skalibs commit that unblocks selfpipe-trapped signals when not
using signalfd.
 - No such block is needed for kbrequest, because the default action
for SIGWINCH is ignore, not terminate.

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