lttng cli: Accept human readable sizes for --subbuf-size

lttng cli: Accept human readable sizes for --subbuf-size

--subbuf-size accepts sizes such as:

- 123 -> 123
- 123k -> 123 * 1024
- 123M -> 123 * 1024^2
- 123G -> 123 * 1024^3

It uses the new parse_size_suffix function, which could probably be used
at other places, such as tracefile size.

Unit tests are included.

Reviewed-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>