feat: C SSHNPD --monitor-read-timeout flag

[JeremyTubongbanua]

- What I did

• Implemented --monitor-read-timeout option in C SSHNPD
• Defaults to 10 seconds

How it works:

Let's say the monitor read timeout is 10 seconds.

Then the monitor connection will do a blocking read on the network buffer for 10 seconds before giving up. Once it gives up, it will send noop:0\r\n to do a check if the monitor connection is alive.

With this new implementation, the monitor connection will only send a noop:0\r\n every monitor-read-timeout seconds, at most.

- Description for the changelog
feat: C SSHNPD --monitor-read-timeout flag

[JeremyTubongbanua]

I added the default constant to sshnpd.h @XavierChanth
Is that the correct place?

[XavierChanth]

Most of the changes were in at_c where I was able to handle the socket code. It turns out mbedtls does have handlers for socket closed, but they shouldn't fully be relied on. We do check for them now, but there are also scenarios where the monitor connection is dead and the socket is still considered open, hence we have configured noop to 45 seconds in this PR.

This noop is only called if the connection has silently failed since the frequency of stats notifications is more often.

[XavierChanth]

I added the default constant to sshnpd.h @XavierChanth Is that the correct place?

It is the correct place, but I renamed it to main.h to be more appropriate.