alfred-monitoring-proxy: Improve curl call

This improves the curl call by:
- Using full executable path
- Removing redundant -X POST
- Using --data-binary instead of --data

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Christian Dresel <fff@chrisi01.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>