Add PID of the cta-taped processes to the output of cta-admin --json dr ls

With tape servers connected to multiple drives, we need to be able to easily distinguish which process is using which drive:

[vlado@tpsrv520 /]$ pstree -p | grep cta | grep master
           |-cta-tpd-master(2667)-+-cta-tpd-IBMLIB1(2532606)-+-{cta-tpd-IBMLIB1}(2532607)
           |-cta-tpd-master(2043843)-+-cta-tpd-IBMLIB1(2532706)-+-{cta-tpd-IBMLIB1}(2532707)

Currently, the --json output of cta-admin dr ls does not contain PIDs.

[vlado@tpsrv520 /]$ cta-admin --json dr ls IBMLIB1-LTO9-F06C3R3 | jq . | grep -i pid
[vlado@tpsrv520 /]$ cta-admin --json dr ls IBMLIB1-LTO9-F06C3R3 | jq . | grep -i 2532606
[vlado@tpsrv520 /]$ cta-admin --json dr ls IBMLIB1-LTO9-F06C3R3 | jq . | grep -i 2532706

Please add it, in particular the PIDs of the processes serving the drives and doing the transfers.