Issue #20: Improve socket name handling

The recently added cmp_sockaddr() function does not call stat(2) any
more to compare local AF_UNIX socket address paths, so not it is stable
enough to use for ordering sockaddr structs.

New function: real_sockaddr() converts the file path of a local AF_UNIX
file socket using realpath(3).  The MDP client uses it on the sender
address of every MDP reply packet it receives to ensure that symlinks in
the instance path do not cause MDP client failures.

Rename recently added socket_setname() function: make_local_sockaddr().
Issue #20: remove unique socket names from tests

The recent work on abstract local socket names means that every instance has
its own unique abstract socket name anyway, so there is no need for test
scripts to use $TFWUNIQUE in MDP and Monitor socket names to avoid
intereference when run concurrently.