pick_interface / pick_kernel_iface lived in _diaglib plus a private copy in wep_lab. Move
them to one scripts/dev.py that any script imports. Consumers (sweep, beacon_watch,
baseline_wifit3, baseline_linux, wep_lab) now import from dev; _diaglib keeps only the
reference-AP loaders. Hardened pick_interface to return None on empty ifaces (was an
IndexError; wep_lab's caller relies on the None). Picker docstrings trimmed to <=2 lines.
Step 1 of retiring _diaglib and splitting scripts/diag.