collect: making some protocol parameters configurable by COLLECT_CONF_

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.

IMHO it is better if these can be set by the application designer, e.g. in a project-conf.h
To increase the value of some of these parameters, the number of bits allocated in the
header should also be changed, so I have extracted those parameters as well. It is not
a standard header and there is no interoperability requirement, so I don't think this is an
issue.

Signed-off-by: Csaba Kiraly <kiraly@disi.unitn.it>