Improve the ability to cancel an in-flight request by using an
interrupt, via SIGCONT, to force the read or write system call to
return prematurely.

Reviewed by:	grehan


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274076 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f