For safety reason, ctrld will create a backup of the current binary when
running upgrade command.
However, on systems where ctrld status is got by parsing ps command
output, the current binary path is important and must be the same with
the original binary. Depends on kernel version, using os.Executable may
return new backup binary path, aka "ctrld_previous", not the original
"ctrld" binary. This causes upgrade command see ctrld as not running
after restart -> upgrade failed.
Fixing this by recording the binary path before creating new service, so
the ctrld service status can be checked correctly.
On some Merlin routers, due to ntp bug, after rebooing, dnsmasq config
was restored to default without ctrld changes, causing ctrld stop
working. Workaround this problem by catching restart diskmon event,
which is triggered by ntpd_synced, then restart dnsmasq.
