libsystemd: make sure we prefix even the dirty secrets in our API with "_sd_"

libsystemd: make sure we prefix even the dirty secrets in our API with "_sd_"

This renames __useless_struct_to_allow_trailing_semicolon__ everywhere
to _sd_useless_struct_to_allow_trailing_semicolon_, to follow our usual
rule of prefixing stuff from public headers that should be considered
internal with "_sd_".

While we are at it, also to be safe: when the struct is used in the C++
protector macros make sure to use two different names depending on
whether it appears in the C++ or C side of things. After all, there
might be compilers that don't consider C++ and C structs the same.

See https://github.com/systemd/systemd/pull/2052#discussion_r46067059