scst/usr/stpgd

This is stpgd service called by the SCST core upon receiving SET TARGET
PORT GROUPS command via SCST events subsystem. In turn, stpgd calls
external script to actually modify ALUA state as requested by the SET
TARGET PORT GROUPS command via cluster manager and SCST sysfs. You can
find example of such script in scst_on_stpg file.

Reason why such dual stage approach is used is, because there is no way
from inside the kernel to control execution of external programs and
there is no way to write a service calling IOCTLs on shell.