* netif.stop() moved after setting the transition callback
* send PPP escape sequence if enabled before waiting for transition to
complete
* add newline character before sync() command (after the "+++")
Closes https://github.com/espressif/esp-protocols/issues/692
* private field 'netif' is not used [clang-diagnostic-unused-private-field]
* private field 'instance' is not used [clang-diagnostic-unused-private-field]
* Call to virtual method 'FdTerminal::stop' during destruction bypasses virtual dispatch [clang-analyzer-optin.cplusplus.VirtualCall]
* unused variable 'TAG' [clang-diagnostic-unused-const-variable]
* Null pointer passed as 2nd argument to memory copy function [clang-analyzer-unix.cstring.NullArg]
* Array access (from variable 'data') results in a null pointer dereference [clang-analyzer-core.NullDereference]
The callback should return false only if there is a problem with the received data.
The updated UART Terminal implementation cannot longer clear is callback.
It is DTE's responsibility to clear the callback.
