Update Python bindings and example to match C library functionality

[andypiper]

The Python bindings got badly out-of-sync with the functionality of the C library / Go code.

This PR:

• removes the pinned backlevel version of pybind from the Makefile, and bumps CMake minimum version to 3.5
• binds all the Python functions against the C code generated from the Go source, and adds definitions for anything that was previously missing (src/main.cpp)
• rewrites tailscale/tsnet.py to be a more Pythonic representation of the API, and adds docstrings
• cleans up and improves the examples/echo.py code to use the upgraded bindings (while keeping it aligned with the C sample)