Add the Linux VSOCK transport.

[remijouannet]

Hello,
VSOCK transport allows communication between the Host and guests.
It requires linux on the host and Guest.

the code is mostly inspire from the tipc and vmci.

there is a conflict between VMCI and VSOCK header, i don't know how to treat this issue, do we support having both build ? or do we make exclusion between them ?
Fixes #4491

[bluca]

Thanks for the PR

i don't know how to treat this issue, do we support having both build ?

Yeah they should both work at the same time

[remijouannet]

@bluca i'm not an expert in cpp, i've used preprocessor to rename the conflicted variables between vmci and vsock, i don't know if this is the right way to handle this.

[remijouannet]

Hello @bluca, any chance to get this merge ? or there is anything i should improve ?

[bluca]

In file included from src/ctx.cpp:30:
/usr/include/linux/vm_sockets.h:178:8: error: redefinition of 'struct sockaddr_vm'
  178 | struct sockaddr_vm {
      |        ^~~~~~~~~~~
In file included from src/ctx.cpp:25:
/home/runner/work/libzmq/libzmq/libzmq/builds/coverage/vmci/vmci_sockets.h:506:8: note: previous definition of 'struct sockaddr_vm'
  506 | struct sockaddr_vm {
      |        ^~~~~~~~~~~

[remijouannet]

sorry i was messing around in the wrong branch, it's fixed now