Attempt ping from front- to back-end several times (#1990).

Attempt ping from front- to back-end several times (#1990).

On macOS it seems that the backend sometimes is not started very
quickly.  Adding a long timeout to ping does not work, perhaps
because the backend is not listening.

Trying a few times seems to fix it.

With this fix I saw this log:

Sun May  9 23:02:51 2021: dcpomatic_disk_writer e63a485e23 started
Sun May  9 23:02:51 2021: Entering main loop
Sun May  9 23:02:51 2021: Writer receives command: P

from the backend and

Sun May  9 23:02:45 2021: Could not send ping to writer (attempt 1)
Sun May  9 23:02:47 2021: Could not send ping to writer (attempt 2)
Sun May  9 23:02:49 2021: Could not send ping to writer (attempt 3)

from the front-end, suggesting that the first 3 pings went to /dev/null
and the 4th was heard.