efi-loader: split efi-api.[ch] from efi-loader.[ch]

efi-loader: split efi-api.[ch] from efi-loader.[ch]

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.