mbedtls_net_free shouldn't close the connection

[XavierChanth]

Suggested enhancement

mbedtls_net_free shouldn't close the connection, only free associated memory.
Or more realistically, to avoid breaking changes, a new method which only frees memory without closing the socket should be implemented.

Justification

When you fork a process that has mbedtls_net_context objects in memory then there is no way to safely clean up the mbedtls_net_context in the child process without disrupting the connection in the parent process. I should be able to control the allocated memory in an individual process without it impacting another process.