plugin: handle ENOSYS as not-supported
plugin: handle ENOSYS as not-supported

Allow plugins to return -ENOSYS during registration and handle it as
"not-supported" error. It makes the error messages slightly more useful in
case kernel-support is missing for a particular subsystem.

mapfau/bluez,pkarasev3/bluez,mapfau/bluez,pkarasev3/bluez,pkarasev3/bluez,silent-snowman/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,mapfau/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,ComputeCycles/bluez