rtapi_shmem: retrieve the size of an attached segment.

This was likely an oversight and is needed for introspection, boundary checks,
and a useful Python API to RTAPI shared memory segments.

Fixes #178 .
halmodule: enable attach to existing rtapi shm segment
Merge pull request #179 from mhaberler/rtapi-shmem-size-param

RTAPI: retrieve the size when attaching a shared memory segment

After a discussion with Michael, this API change looks mostly internal.  While it could be used externally in a comp, it looks unlikely, and even more so at this early stage of Machinekit's existence.  Therefore, we're not going to worry much about the 'Evolution of Public Contracts' section of C4 [1].

Merging patch.

[1] http://rfc.zeromq.org/spec:22#toc8