[PR Proposal - Opinions] Improve NSH Kill cmd

[Rodrigo Garcia <rp...@gmail.com>]

Hello all,

I want to submit a PR with an improvement on NSH kill cmd.
The idea is to make it a bit closer to unix kill behavior.

In unix, kill command will assume SIGTERM as the default signal when the
<-SIGNAL> option is omitted.
The PR I want to submit would implement such behavior.
It would have this sintax:  "kill [<-signal>] <pid>"
Assuming SIGTERM as default signal to be sent to PID.

The objective behind it is to allow the user to terminate any [background]
running task just with a single "kill <pid>" and, eventually, to allow the
task to catch <SIGTERM> and terminate gracefully in both CONFIG_SIG_DEFAULT
defined or not.

Is there any restriction related to this PR proposal?

Thanks,
Rodrigo.