Add network-online.target in .service files

This helps the daemon work better on IoT devices
like RaspberryPi.
According to systemd's documentation,
`network.target` means there has already been
a network manager started, but the network may
not be "up". On most PCs this does not matter
because the network will turn to "up" almost
immidiately. The IoT devices' network interface
may not be set up quickly enough, so they may
meet that the sing-box daemon is started before
network is ready, which results that sing-box
cannot find a working route. The workaround
of this is restarting sing-box daemon but it
absolutely is not the perfect solution.
As `network-online.target` must be triggered by
network manager after you configured it, I keep
`network.target` so there will be no change to
those who do not enabled proper trigger service
like `NetworkManager-wait-online.service`.

See also: https://systemd.io/NETWORK_ONLINE/

release/config/sing-box.service

@@ -1,7 +1,7 @@
 [Unit]
 Description=sing-box service
 Documentation=https://sing-box.sagernet.org
-After=network.target nss-lookup.target
+After=network.target nss-lookup.target network-online.target
 
 [Service]
 CapabilityBoundingSet=CAP_NET_ADMIN CAP_NET_BIND_SERVICE CAP_SYS_PTRACE CAP_DAC_READ_SEARCH

release/config/sing-box@.service

@@ -1,7 +1,7 @@
 [Unit]
 Description=sing-box service
 Documentation=https://sing-box.sagernet.org
-After=network.target nss-lookup.target
+After=network.target nss-lookup.target network-online.target
 
 [Service]
 CapabilityBoundingSet=CAP_NET_ADMIN CAP_NET_BIND_SERVICE CAP_SYS_PTRACE CAP_DAC_READ_SEARCH

release/local/sing-box.service

@@ -1,7 +1,7 @@
 [Unit]
 Description=sing-box service
 Documentation=https://sing-box.sagernet.org
-After=network.target nss-lookup.target
+After=network.target nss-lookup.target network-online.target
 
 [Service]
 CapabilityBoundingSet=CAP_NET_ADMIN CAP_NET_BIND_SERVICE CAP_SYS_PTRACE CAP_DAC_READ_SEARCH