tty: n_gsm: add keep alive support

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapters 5.4.6.3.4 and 5.1.8.1.3 describe the test
command which can be used to test the mux connection between both sides.

Currently, no algorithm is implemented to make use of this command. This
requires that each multiplexed upper layer protocol supervises the
underlying muxer connection to handle possible connection losses.

Introduce ioctl commands and functions to optionally enable keep alive
handling via the test command as described in chapter 5.4.6.3.4. A single
incrementing octet "ka_num" is being used for unique identification of each
single keep alive packet. Retries will use the same "ka_num" value as the
original packet. Retry count and interval are taken from the general
parameters N2 and T2.

Add usage description and basic example for the new ioctl to the n_gsm
documentation.

Note that support for the test command is mandatory and already present in
the muxer implementation since the very first version.
Also note that the previous ioctl structure gsm_config cannot be extended
due to missing checks against zero of the field "unused".

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230214122737.1976-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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