"Add estat subcommands"

[brad-lewis]

Replacements are needed for illumos peformance playbook scripts:

For these scripts there are dtrace scripts whose general approach appears that it will carry over on linux. The dtrace just needed to be implement using ebpf and some front end.
arc-prefetch
metaslab-alloc
profile
txg.d
zfs-write

For these there is a dtrace script whose approach appear to be illumos specific. The
equivalent functions or dat fields need to found on illumos as well as rewriting the
script.
comstar-idm-send.d - iscsi xfer-start and xfer-done endpoints (time to get a request ready to send)
comstar-idm-so-send.d - iscsi to write the data, from idm_so_send_buf_region entry to exit
comstar-stmf-io.d - scsi-task-start to scsi-task-end
comstar-stmf-queue.d - iscsi wait times from stmf_itl_task_done
mac.d - vmxnet3_tx
nfs-by-vdb.d
nfs-threads.d
rpc-flowctl.d
rpc-nfs-queue.d
rpc.d
tcp-congestion.d

This one can be Adapted from an stbtrace script/
nfs-by-client.d 33240 Update copyright 5 years ago