start_watchdog should support init system shell

[alesavin]

Now mount.efs/start_watchdog support init systems

• init
• systemd
• launchd
  with default resolving of init systam by cat /proc/1/comm. In case resolving ends with non-supported init system warning is shown and watchdog isn't started. In a system (container) without any option from list above (init, launchd aren't avaliable, systemd requires additional setup) we can't easily track stunnel leftovers.

Proposal is to add support for init system based on raw shell, for such case

• existence of PID file (/var/run/efs/amazon-efs-mount-watchdog.pid) is checked and in case it exists existence of process with PID
• in case we have no file or process

/usr/bin/amazon-efs-mount-watchdog &
echo $! > /var/run/efs/amazon-efs-mount-watchdog.pid

• otherwise report we have watchdog process is already running

[RyanStan]

Thanks for bringing this to our attention!