As tested on Linux:
- An SCM_RIGHTS ancillary message is "attached" to the range of data bytes sent in the same sendmsg() call.
- However, as always, recvmsg() calls on the receiving end don't necessarily map 1:1 to sendmsg() calls. Messages can be coalesced or split.
- The recvmsg() call that receives the first byte of the ancillary message's byte range also receives the ancillary message itself.
- To prevent multiple ancillary messages being delivered