s6-notify: support fd-based s6 notification protocol

s6 notification protocol is a simple protocol to notify the supervisor
when the daemon is ready to serve its purpose.

Just write a newline to the fd specified by the "S6_NOTIFY_FD" env-var
and close it, and then the supervisor marks the daemon as ready.

Dinit and s6 supervision suite are known to support this protocol.

Tested with three cases:
1. "copyparty-sfx.py -c config.conf" and everything works just as before.
2. "copyparty-sfx.py -c config.conf" with S6_NOTIFY_FD when using Dinit
   and its "ready-notification = pipevar:S6_NOTIFY_FD" option and
   copyparty notifies the supervisor when it's ready.
3. "copyparty-sfx.py -c config.conf" with S6_NOTIFY_FD=3 but without the
   fd 3 being open which logs a warning from s6-notify.

See https://skarnet.org/software/s6/notifywhenup.html and
https://davmac.org/projects/dinit/man-pages-html/dinit-service.5.html#ready
for details regarding its usage.

Signed-off-by: Mobin Aydinfar <mobin@mobintestserver.ir>

copyparty/svchub.py

@@ -987,6 +987,10 @@ class SvcHub(object):
 
         Daemon(self.sd_notify, "sd-notify")
 
+        zb = os.environ.get("S6_NOTIFY_FD")
+        if zb:
+            Daemon(self.s6_notify, "s6-notify", (zb,))
+
     def _feature_test(self) -> None:
         fok = []
         fng = []
@@ -1897,6 +1901,17 @@ class SvcHub(object):
         except:
             self.log("sd_notify", min_ex())
 
+    def s6_notify(self, zb: bytes) -> None:
+        try:
+            fd = int(zb)
+            if fd < 3:
+                raise Exception("value < 3")
+            os.write(fd, b"\n")
+            os.close(fd)
+        except:
+            t = "S6_NOTIFY_FD=%s:\n%s"
+            self.log("s6-notify", t % (zb, min_ex()), 1)
+
     def log_stacks(self) -> None:
         td = time.time() - self.tstack
         if td < 300: