Adds a new "overlapdial" configuration parameter that enables
incoming overlap dialing when set to "incoming", "yes" or "both" (possible values:
"no", "yes"/"both", "incoming"/"receive", "outgoing"/"send").

(Overlap dialing is disabled by default)

NOTE: only the non-overlap receive case has been tested (= doesn't break existing setups)
Signed-off-by: Stefan Knoblich <s.knoblich@axsentis.de>