simple_talker: bugfix, MRP func returns 1500 for success

Bug introduced by coverity fixes. This historical API goes
against the principle of least surprise, to be discussed.
For now, this commit will bring back 2 coverity defects but
workaround the bug.
MRP client code talker side: except for send_mrp_msg(), fincs return 0 or -1

Callers of send_mrp_msg() are able to ckeck the message length returned.
Callers of higher level functions get 0 for sucess, -1 for failure.
MRP client code listener side: except for send_msg(), fincs return 0 or -1

Callers of send_msg() are able to ckeck the message length returned.
Callers of higher level functions get 0 for sucess, -1 for failure.
talkers: mrp_unadvertise_stream is called at the end, if fail dont exit just warn
Merge pull request #163 from jbmaillet/bugfix

simple_talker: bugfix, MRP func returns 1500 for success + minor mrp msg function API changes.