dahdi_cfg: Allow dynamic spans to handle udev based span assignment.

Prior to this commit, if auto_assign_spans was set to 0, it was possible to
get in an endless loop creating and destroying spans. The primary reason was
that all dynamic spans are destroyed and recreated each time dahdi_cfg runs, BUT
dahdi_cfg was run each time a new span showed up in udev when auto_assign_spans
was set to 0.

Now dahdi_cfg will only destroy and recreate dynamic spans if dahdi_cfg is run
without a specifc span number. Also, while creating dynamic spans, dahdi_cfg
will pause for up to one second for the span to be assigned in order to ensure
that the spans are numbered consistently when auto span assignment is
configured.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>