mwan3: run user scripts in backgorund (#992)

/etc/mwan3.user should finish in a timely manner.
(https://openwrt.org/docs/guide-user/network/wan/multiwan/mwan3#alertsnotifications)
As of today, the only script is send-mwan-alert which can take 3
minutes.

#1014

Author: filippocarletti

packages/ns-plug/files/40_ns-plug_mwan_hooks

@@ -1,2 +1,3 @@
 # setup mwan3 custom scripts
-grep -q '/usr/libexec/ns-plug/mwan-hooks' /etc/mwan3.user || echo '/usr/libexec/ns-plug/mwan-hooks' >> /etc/mwan3.user
+grep -q '/usr/libexec/ns-plug/mwan-hooks &' /etc/mwan3.user || echo '/usr/libexec/ns-plug/mwan-hooks &' >> /etc/mwan3.user
+sed -i 's#^/usr/libexec/ns-plug/mwan-hooks$#/usr/libexec/ns-plug/mwan-hooks \&#' /etc/mwan3.user