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.

src/unix/process.c

@@ -73,7 +73,9 @@ static void uv__chld(uv_signal_t* handle, int signum) {
   assert(signum == SIGCHLD);
 
   for (;;) {
-    pid = waitpid(-1, &status, WNOHANG);
+    do
+      pid = waitpid(-1, &status, WNOHANG);
+    while (pid == -1 && errno == EINTR);
 
     if (pid == 0)
       return;