It provides several important constants, especially _PATH_BSHELL, which
is used in PID1, executor, and run. The header has been included
indirectly through e.g. libmount.h, mntent.h, utmpx.h, and so on.
Let's explicitly include it in forward.h, as libmount.h and friends that
includes paths.h are irrelevant to _PATH_BSHELL, and we may easily fail
to build when code is touched.
The header is not heavy, hence should not hurt anything.
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
Mike Yuan
Yu Watanabe
Daan De Meyer