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.
Some refactoring: split efi-loader.[ch] in two: isolate the calls that
implement out boot loader interface spec, and those which implement
access to upstream UEFI firmware features.
They are quite different in nature and behaviour, and even semantically
it makes to keep these two separate. At the very least because the
previous name "efi-loader.[ch]" suggests all was about loader-specific
APIs, but much of it is generic uefi stuff...
While we are at it, I renamed a bunch of return parameters to follow our
usual ret_xyz naming. But besides renaming no real code changes.