Introduce forward.h header with forward declarations (#37428)

In preparation for adopting forward declarations to reduce unnecessary
transitive includes across the tree, let's introduce a forward.h header
with forward declarations for all libc, libsystemd, basic and shared
types.

Additionally, this header exports all basic integer types and errno
constants, as well as all macros including assertions macros. These
header files contain types often used in headers and are always included
in every source file one way or another anyway.

To avoid having to include memory-util.h and alloc-util.h in forward.h,
we split off the parts we need from both into cleanup-util.h and only
include cleanup-util.h in forward.h.

To keep this commit self-contained, we include cleanup-fundamental.h and
cleanup-util.h from the headers that originally contained the same
macros. We'll remove these again in a later commit that optimizes the
includes in src/basic and src/fundamental.

Split out of #37364