Allow `sendmsg` to accept a preallocated `Vec`

[vkrasnov]

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Currently sendmsg allocates a new vector for cmsg_buffer every time it is called. It would be nice if it could accept a mutable reference to Vec instead so it can reuse existing storage instead or &mut [u8], perhaps as an option?

[SteveLauC]

Hi, it would be nice to avoid heap allocations here, but if we allow users to pass a pre-allocated cmsg buffer, then users have to ensure that:

1. ~~The buffer is zero-initialized~~
2. The buffer is sufficient in size

~~For 1, Nix can guarantee this by (possibly) re-initializing it.~~ For 2, if we use a vector and it is insufficient, we can re-allocate to extend the capacity.

Perhaps we should provide an alternative interface? Something like sendmsg_pre_alloc()?

[SteveLauC]

The buffer is zero-initialized

Cannot remember why I said this is needed, I think we do not need to satisfy this condition.