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

[yamt]

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

• I have verified before submitting the report.