unix: retry waitpid() on EINTR

Before this commit, libuv would abort() if waitpid() failed with EINTR.

It's unlikely that anyone actually hit this error condition: the major
UNIX platforms - with the possible exception of Solaris - don't return
EINTR when the WNOHANG flag is specified, as libuv does.

However, POSIX allows for an implementation to do whatever here: unless
explicitly forbidden, it's allowed and POSIX doesn't restrict
implementers in this particular area.

Let's opt for robustness and handle EINTR.