MEDIUM: cli: Add support for dynamically allocated payloads

MEDIUM: cli: Add support for dynamically allocated payloads

It is now possible to deal with too big payload to fit in a buffer, without
changing the buffer size. By default, a payload up to 128 KB can be
dynamically allocated. "tune.cli.max-payload-size" global parameter can be
used to change this value, with some caution for huge values.

For CLI command handler functions, there is no change at all. A pointer on
the payload is still passed as parameter. Internally, an area is allocated
for the payload only if it is too big.

The payload pattern used to detect the end of the payload is part from the
allocated area.