Builds and installs curve_keygen tool
Fixed overwrite in zmq_getsockopt

On ZMQ_CURVE_xxxKEY fetches, would return 41 bytes into caller's 40-byte
buffer. Now these fetches only return 41 bytes if the caller explicitly
provides a 41-byte buffer (i.e. the option size is 41).
Merge pull request #655 from hintjens/master

Fixed overwrite in zmq_getsockopt