These sockets don't handle multipart data, so if callers send it,
they drop frames, and things break silently.
Solution: if the caller tries to use ZMQ_SNDMORE, return -1 and
set errno to EINVAL.
Of course people still "can" distributed the sources under the
LGPLv3. However we provide COPYING.LESSER with additional grants.
Solution: specify these grants in the header of each source file.
