failNoPeers context support.

fixes nanomsg#224 Desire FastSendFailNoPeer option

Shut up codecov (thanks for breaking paths codecov!)

Fix compilation errors in 3.1.1 on Windows.

Make IPC option symbols available everywhere.

This reduces some friction when using these options in
code that may be built for different platforms.

fixes nanomsg#213 Want options to set permissions on IPC socket

This adds new options only available on UNIX systems to set
the permissions and ownership of the UNIX domain socket.
It also adds fixes to work better when testing with WSL
by forcing the sockets to be created in a temp dir.

fixes nanomsg#201 Requests respond with send timeout after some time

* Add test demonstrating req ctx send/close race
* Add pipe back to ready queue after send unless the socket is closed, rather than the context

Backport nanomsg#195 slowest rep worker blocks faster workers to v2

Version 3.0.1

Version 3.0.0

Note that this introduces a new import path!

Please change all references from nanomsg.org/go/mangos/v2 to
the new path: go.nanomsg.org/mangos/v3