chan_dahdi: Fix erroneously persistent dialmode.

chan_dahdi: Fix erroneously persistent dialmode.

It is possible to modify the dialmode setting in the chan_dahdi/sig_analog
private using the CHANNEL function, to modify it during calls. However,
it was not being reset between calls, meaning that if, for example, tone
dialing was disabled, it would never work again unless explicitly enabled.

This fixes the setting by pairing it with a "perm" version of the setting,
as a few other features have, so that it can be reset to the permanent
setting between calls. The documentation is also clarified to explain
the interaction of this setting and the digitdetect setting more clearly.

Resolves: #1378
(cherry picked from commit 9b5ffe513d7ffe2cf6de203e03c0c98b80c1a7ac)