collect: making some protocol parameters configurable by COLLECT_CONF_ style defines

TTL (which has a rather low default of 15), MAC level retransmissions,
overall number of retransmissions, and the header bits dedicated to these
were all fixed in the collect.h and collect.c, without a simple way to
override them.
Extracted these as COLLECT_CONF_ parameters, keeping defaults as they were
before.

Signed-off-by: Csaba Kiraly <kiraly@disi.unitn.it>
Merge pull request #538 from cskiraly/collect-fixes

collect: making some protocol parameters configurable by COLLECT_CONF_