Allow sendmmsg to carry an extra payload added to every msg

[nmldiegues]

Add sendmmsg_with_suffix, a variant of sendmmsg that appends a
caller-supplied suffix (e.g. shared framing) to every datagram in the
batch using an extra iovec per message, avoiding a copy into a
combined buffer.

Fix two latent bugs in sendmmsg along the way:

• A partial batch failure after some messages had already been sent
  would discard the last OS error and silently report success.
• The early-exit-on-short-write check compared against MAX_MMSG
  instead of the current chunk's length, so the last (possibly
  shorter) chunk never triggered early exit.
• Zero-length input now returns Ok(0) instead of hitting msgvec
  with no capacity.

iovecs is fully populated before any mmsghdr.msg_iov pointers are
taken, per the safety invariant documented in mmsg.rs/AGENTS.md,
preventing dangling pointers from a SmallVec reallocation.

Renames the sendmmsg test to send_many and adds coverage for the
new suffix path, empty batches, and error propagation without
progress.