ABI refactoring: Rename event counter ABI to "old"

ABI refactoring: Rename event counter ABI to "old"

The counter ioctls exposed in 2.13 for notifications use a fixed-size
array for the number of dimensions.

We aim to replace this by variable length arrays in 2.14 for event
counters, and introduce new ABIs as extensible structures.

Rename the ioctls as follows:

- LTTNG_KERNEL_ABI_COUNTER -> LTTNG_KERNEL_ABI_OLD_COUNTER
- LTTNG_KERNEL_ABI_COUNTER_READ -> LTTNG_KERNEL_ABI_OLD_COUNTER_READ
- LTTNG_KERNEL_ABI_COUNTER_AGGREGATE -> LTTNG_KERNEL_ABI_OLD_COUNTER_AGGREGATE
- LTTNG_KERNEL_ABI_COUNTER_CLEAR -> LTTNG_KERNEL_ABI_OLD_COUNTER_CLEAR

And rename the following structures:

- lttng_kernel_abi_counter_dimension -> lttng_kernel_abi_old_counter_dimension
- lttng_kernel_abi_counter_conf -> lttng_kernel_abi_old_counter_conf
- lttng_kernel_abi_counter_index -> lttng_kernel_abi_old_counter_index
- lttng_kernel_abi_counter_value -> lttng_kernel_abi_old_counter_value
- lttng_kernel_abi_counter_read -> lttng_kernel_abi_old_counter_read
- lttng_kernel_abi_counter_aggregate -> lttng_kernel_abi_old_counter_aggregate
- lttng_kernel_abi_counter_clear -> lttng_kernel_abi_old_counter_clear

Introduce "TODO" comments where the new ABIs should be implemented.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I7a4f04e89a56ca1415d326976c4ea3afb9715295