Let's not leak details from src/shared and src/libsystemd into
src/basic, even though you can't actually do anything useful with
just forward declarations from src/shared.
The sd-forward.h header is put in src/libsystemd/sd-common as we
don't have a directory for shared internal headers for libsystemd
yet.
Let's also rename forward.h to basic-forward.h to keep things
self-explanatory.
As suggest here:
https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax
"You may optionally specify attribute names with ‘__’ preceding and
following the name. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use the attribute name __noreturn__ instead of noreturn. "
It also used __bitwise and __force. It seems easier to rename
our versions since they are local to this one single header.
Also, undefine them afteerwards, so that we don't pollute the
preprocessor macro namespace.
Daan De Meyer
Lennart Poettering
Yu Watanabe
Zbigniew Jędrzejewski-Szmek
Lubomir Rintel
Kay Sievers