syncthing: enable via config, lower priority, cleanup

syncthing: enable via config, lower priority, cleanup

Added the ability to enable/disable via a config file;
the default is enabled for backwards compatibility.

This is an additional service, so running it the
same priority as dnsmasq, etc does not make sense.

Added USE_PROCD=1

"done" starts at "START=95", so this should start earlier

Added STOP=10 to stop syncthing early

Compile-tested on: ipq806x, ipq40xx
Runtime-tested on: ipq806x, ipq40xx

Signed-off-by: Marc Benoit <marcb62185@gmail.com>
(cherry picked from commit 6f6877460a775a75f16154f2d75944bf91770f0d)