Rework the wireless control panel section to respect non-wireless connec...

...tions.

Fixes #4410    https://bugs.sugarlabs.org/ticket/4410

The 'Discard network history' button is peculiar to Sugar, and
is destructive to all non-wireless connections, despite being in
the 'Wireless' section of the control panel.

Relabel this button and code variables to be specific to wireless
connections.

Recode tests for non-sugar-wireless connections to exclude
non-wireless connections.

Relabel the 'Radio' checkbox and instructions to avoid confusion
over its state and action.

Test for the availability of wireless hardware with the kernel's
sys filesystem before building and showing the 'Wireless' section
on each invocation of the 'Network' panel.