Trust signalfd on illumos, again.

Trust signalfd on illumos, again.

Commit 3ab4fc5d avoided choosing signalfd by default on illumos, because
it triggered kernel panics.  That was fixed, so we can remove a kludge
from our code.  Users/packagers can still override the default choice at
compile time if desired, and we'll leave the back-branches unchanged so
they keep choosing self-pipe by default, but we'll default to signalfd
(like we do for Linux) in 17.  Fixed kernels should be everywhere by the
time 17 ships.

The illumos issues were:

 * https://www.illumos.org/issues/13700
 * https://www.illumos.org/issues/14892

Discussion: https://postgr.es/m/CA+hUKG+NK-K_G_i1H3OpDTwYPEsiwQi_jw58PGcW2H+-N2eVCA@mail.gmail.com