MINOR: cli: add "kill" CLI command

MINOR: cli: add "kill" CLI command

Implements a new "kill" admin CLI command. This allows stopping other
running commands.

This patch is implemented in two parts. First, the "kill" parser, which
loops over CLI jobs and sets the CLI_CMD_FL_KILLED flag on the targeted
ones.

The other change is in cli_io_handler() so that each targeted job reacts
to the CLI_CMD_FL_KILLED flag. If set, normal CLI flow is interrupted
and the io_release handler is called. Any release pending message is
suppressed and "Killed." is displayed to notify the user.

Currently, "kill" can only stop uninterruptible commands. This must
still be specified as a mandatory argument to support future extensions
of the command.