sigwait() cancellation point

[David Xu]

Jilles Tjoelker wrote:
> Recent versions of glibc do this. They implement
> sigwait/sigwaitinfo/sigtimedwait based on a single extended sigtimedwait
> system call; sigwait differs from the others by retrying when it gets
> EINTR. This is not documented very well in man pages (for example, man
> sigwait might get you an ancient LinuxThreads man page).
> 
> One reference:
> http://lkml.indiana.edu/hypermail/linux/kernel/0508.0/0181.html
> 
> Google also provides various examples of applications that treated any
> sigwait() error as fatal and needed to be changed to work reliably on
> systems where sigwait() may return EINTR (not just FreeBSD).
> 
I have committed it, thanks!