linux: Set timeout argument on poll() properly

`0` for the timeout means *return immediately* instead of
*wait forever*. This is absolutely unnecessary, so change
the argument to `-1` which does mean *wait until something happens*.

linux/esptun.c

@@ -540,7 +540,7 @@ int main(int argc, char **argv)
             fds[i].revents = 0;
         }
 
-        if (poll(fds, 2, 0) < 0) {
+        if (poll(fds, 2, -1) < 0) {
             perror("poll");
             exit(1);
         }

linux/term.c

@@ -92,7 +92,7 @@ int main(int argc, char **argv) {
             fds[i].revents = 0;
         }
 
-        if (poll(fds, 2, 0) < 0) {
+        if (poll(fds, 2, -1) < 0) {
             fprintf(stderr, "Poll error: %s\n", strerror(errno));
             exit(1);
         }