MonitorFdHup: raise explicit SysError rather unreachable

Syscalls can fail for many reasons and we don't want to loose the errno
and error context.

(cherry picked from commit 8e0bc2c3a8)

src/libutil/unix/monitor-fd.hh

@@ -39,8 +39,11 @@ public:
 #endif
                     ;
                 auto count = poll(fds, 1, -1);
-                if (count == -1)
-                    unreachable();
+                if (count == -1) {
+                    if (errno == EINTR || errno == EAGAIN)
+                        continue;
+                    throw SysError("failed to poll() in MonitorFdHup");
+                }
 
                 /* This shouldn't happen, but can on macOS due to a bug.
                    See rdar://37550628.