BUG/MEDIUM: cli: always reset the service context between commands

BUG/MEDIUM: cli: always reset the service context between commands

Erwan Le Goas reported that chaining certain commands on the CLI would
systematically crash the process; for example, "show version; show sess".
This happened since the conversion of cli context to appctx->svcctx,
because if applet_reserve_svcctx() is called a first time for a tiny
context, it's allocated in-situ, and later a keyword that wants a
larger one will see that it's not null and will reuse it and will
overwrite the end of the first one's context.

What is missing is a reset of the svcctx when looping back to
CLI_ST_GETREQ.

This needs to be backported to 2.6, and relies on previous commit
"MINOR: applet: add a function to reset the svcctx of an applet".