Update our copy of sha1dc to the upstream commit 855827c (Detect
endianess on HP-UX, 2019-05-09). Changes include fixes to endian
detection on AIX and HP-UX systems as well as a define that
allows us to force aligned access, which we're not using yet.

Users of the Win32 hash cannot be inlined, as it uses a static struct.
Don't inline it, but continue to declare the function in the header.

This means the forward declaration isn't necessary.  The forward
declaration can cause compilation errors as it conflicts with the
`GIT_INLINE` declaration (the signatures are different).

Move to the `git_error` name in the internal API for error-related
functions.

The `git_hash_global_init` function is simply defined as a macro to zero
for most of the different hash implementations. This makes it impossible
to treat it like a function pointer, which is required for a later
commit where we want to improve the way global initialization works.
Fix the issue by converting all no-op macros to an inline function
returning zero.

There's a small gotcha here, though: as most hash implementations only
have a header file, but not a corresponding implementation file, we
cannot declare the function as non-static. But declaring it as `static
inline` fails, too, as there is a previous declaration as non-static. So
we have to move the function declaration after the include that brings
in the function definition, as it is allowed to have a non-static
declaration after a static definition, but not the other way round.

This updates the version of SHA1DC to c3e1304ea3.

use consistent names for the #include / #define header guard pattern.