getentropy(3): Trap non-API errnos from getrandom(2) and abort

Additionally, reconcile our abort behavior with arc4random(3). Unlike
SIGABRT, SIGKILL cannot be caught by the user program. These failures
are fatal conditions and should not return to the caller, as they did in
the instance that resulted in D17049.

While here, fix some minor typos in a comment.

Reviewed by: delphij
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17050