Clears all the sessions in a given tunnel type. When
reading this object, the value of 'none' will always be
returned.
When setting these values, the following operations will be
performed:
none: no operation.
all: clears all the sessions in all the tunnels.
l2f: clears all the L2F sessions.
l2tp: clears all the L2TP sessions.
pptp: clears all the PPTP sessions.