community/[various]: Add group and use it as primary in .pre-* scripts.

Fixes the problem I unintentionally brought in commit ccc056db:
system user creation doesn't add same named group and uses nogroup as
primary group unless explicitly specified via -G.

Brings status quo regarding primary groups of users created in packages:
- community/caddy
- community/domoticz
- community/oscam
- community/syncthing

community/caddy/caddy.pre-install

 #!/bin/sh
 
-adduser -S -D -h /var/lib/caddy -s /sbin/nologin -g caddy caddy 2>/dev/null
+addgroup -S caddy 2>/dev/null
+adduser -S -D -h /var/lib/caddy -s /sbin/nologin -G caddy -g caddy caddy 2>/dev/null
 adduser caddy www-data 2>/dev/null
 
 exit 0

community/domoticz/domoticz.pre-install

 #!/bin/sh
 
-adduser -S -D -h /var/lib/domoticz -s /sbin/nologin -g domoticz domoticz 2>/dev/null
+addgroup -S domoticz 2>/dev/null
+adduser -S -D -h /var/lib/domoticz -s /sbin/nologin -G domoticz -g domoticz domoticz 2>/dev/null
 
 exit 0

community/oscam/oscam.pre-install

 #!/bin/sh
 
-adduser -S -D -H -s /sbin/nologin -g oscam oscam 2>/dev/null
+addgroup -S oscam 2>/dev/null
+adduser -S -D -H -s /sbin/nologin -G oscam -g oscam oscam 2>/dev/null
 addgroup oscam usb 2>/dev/null
 
 exit 0

community/syncthing/syncthing.pre-install

 #!/bin/sh
 
-adduser -S -D -H -h /var/lib/syncthing -s /sbin/nologin -g syncthing syncthing 2>/dev/null
+addgroup -S syncthing 2>/dev/null
+adduser -S -D -H -h /var/lib/syncthing -s /sbin/nologin -G syncthing -g syncthing syncthing 2>/dev/null
 
 exit 0