io-util: introduce loop_write_full that takes a timeout

Also drop do_poll as the use case is covered
by timeout.

src/basic/random-util.c

@@ -223,7 +223,7 @@ int random_write_entropy(int fd, const void *seed, size_t size, bool credit) {
                 if (ioctl(fd, RNDADDENTROPY, info) < 0)
                         return -errno;
         } else {
-                r = loop_write(fd, seed, size, false);
+                r = loop_write(fd, seed, size);
                 if (r < 0)
                         return r;
         }