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.

hash: openssl: check return values of SHA1_* functions

The OpenSSL functions `SHA1_Init`, `SHA1_Update` and `SHA1_Final` all
return 1 for success and 0 otherwise, but we never check their return
values. Do so.

Teach the CommonCrypto hash mechanisms to support large files.  The hash
primitives take a `CC_LONG` (aka `uint32_t`) at a time.  So loop to give
the hash function at most an unsigned 32 bit's worth of bytes until we
have hashed the entire file.

Teach the win32 hash mechanisms to support large files.  The hash
primitives take at most `ULONG_MAX` bytes at a time.  Loop, giving the
hash function the maximum supported number of bytes, until we have
hashed the entire file.