_system_properties.h: adding PROP_MSG_GETPROP and PROP_MSG_LISTPROP

Signed-off-by: Ricardo Salveti de Araujo <ricardo.salveti@canonical.com>
Signed-off-by: Sergio Schvezov <sergio.schvezov@canonical.com>
Merge pull request #1 from sledges/fix-getprop

adding support for getprop and listprop