[libc] Add struct cmsghdr and associated macros (#193756)
The macros are the main source of subtlety. The interesting aspects are:
- some implementations CMSG_ALIGN the size of struct cmsghdr, but this
is a noop. Instead of doing that, I added an assertion in the test.
- POSIX permits CMSG_NXTHDR to return null if the buffer has no space
for the data array, and this behavior differs between implementations.
This implementation does not do that in order to match CMSG_FIRSTHDR,
which doesn't have such an option.
- some implementations redirect the CMSG_NXTHDR macro to an (extern or
static inline) function. I implemented this inside the macro to avoid
having to define a (private ?) entry point for that function.
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Co-authored-by: Jeff Bailey <jbailey@raspberryginger.com>
