Add Context.lun_reset() to expose iscsi_task_mgmt_lun_reset_sync

[bmeagherix]

Expose iscsi_task_mgmt_lun_reset_sync as Context.lun_reset(lun).

iscsi_task_mgmt_lun_reset_sync has been part of libiscsi for more than a decade. The new method follows the same error-handling convention as the rest of the Context API: returns None on success, raises RuntimeError on failure.

[bmeagherix]

Tested by issuing a LUN RESET against a live iSCSI target and confirming the Logical Unit Reset PDU on the wire via tcpdump.

import iscsi

ctx = iscsi.Context("<initiator-iqn>")
ctx.set_targetname("<target-iqn>")
ctx.set_session_type(iscsi.iscsi_session_type.ISCSI_SESSION_NORMAL)
ctx.connect("<target-ip>:3260", 0)
print("connected")
ctx.lun_reset(0)
print("LUN RESET OK")
ctx.disconnect()
print("disconnected")

[rosjat]

@bmeagherix if you feel confident just merge the change. Im not sure if @Flameeyes will review any time soon