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This removes the duplicate code in cyw43, esp32, esp8266 that implements
the same logic as network.hostname.

Renames the `mod_network_hostname` (where we store the hostname value in
`.data`) to `mod_network_hostname_data` to make way for calling the shared
function `mod_network_hostname`.

And uses memcpy for mod_network_hostname_data, because the length of source
is already known and removes reliance on string data being null-terminated.

This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>