This change is in preparation to [PR 10788]. PR 10788 will make the
shell exitable which may lead to unexpected behavior in comparison to
previous usage of the shell.
To prevent this, this PR introduces two "new" functions to the shell's
API: `shell_run_once()` and `shell_run_forever()`.
`shell_run_once()` basically has the same behavior as `shell_run()` in
current master: Start a shell and continue reading lines until EOF is
reached.
`shell_run_forever()` wraps around `shell_run_once()` and restarts the
shell if it exits.
`shell_run()` is re-introduced as a back-porting alias for
`shell_run_forever()`.
As a consequence all current calls to `shell_run()` won't exit even
with [PR 10788] merged (which would add EOT as additional exit
condition for `shell_run_once()`).
[PR 10788]: https://github.com/RIOT-OS/RIOT/pull/10788
2c1a2863ce