[BUG] msg_iovlen == 1 assumptions in sendmsg/recvmsg

### Description / Steps to reproduce the issue

* inet_sendmsg handles msg_iovlen != 1 cases by copying to a temporary buffer. it isn't ideal.
* otherwise, in most of places, msg_iovlen != 1 for sendmsg/recvmsg seems unimplemented.

### On which OS does this issue occur?

[OS: Mac]

### What is the version of your OS?

macOS 15.2

### NuttX Version

master

### Issue Architecture

[Arch: all]

### Issue Area

[Area: Networking]

### Verification

- [x] I have verified before submitting the report.

Start by tracing inet_sendmsg and the other sendmsg/recvmsg paths mentioned in the report, focusing on how msg_iovlen != 1 is handled. Compare the existing temporary-buffer behavior with paths that assume msg_iovlen == 1; done means the affected cases are implemented consistently for all architectures and verified on macOS.

Written by the indexing model from the issue text.