Win32 fixes for diff/checkout/reset

Add option to ignore file mode in diffs

Diff API signatures

A number of diff APIs and the `git_checkout_index` API take a
`git_repository` object an operate on the index.  This updates
them to take a `git_index` pointer explicitly and only fall back
on the `git_repository` index if the index input is NULL.  This
makes it easier to operate on a temporary index.

The index iterator could previously only be created from a repo
object, but this allows creating an iterator from a `git_index`
object instead (while keeping, though renaming, the old function).

The diff API is not in the parameter order one would expect from
other libgit2 APIs.  This fixes that.

The reset hard tests had hardcoded expected file content and was
not correctly compensating for CRLF filtering when a file needed
to be reverted by the reset hard.  This fixes that.

The existing p_lstat implementation on win32 is not quite POSIX
compliant when setting errno to ENOTDIR.  This adds an option to
make is be compliant so that code (such as checkout) that cares
to have separate behavior for ENOTDIR can use it portably.

This also contains a couple of other minor cleanups in the
posix_w32.c implementations to avoid unnecessary work.

Using the builtin strcmp and strcasecmp as function pointers is
problematic on win32.  This adds internal implementations and
divorces us from the platform linkage.

Explain a few CMake options in the README

Expose STDCALL and explain a few useful CMake options, as well as
explain the quirks building on Windows.

Allocate with calloc rather than conditionally memsetting a specific
part of the struct later on.

Deal with empty and nonexsitent values in config

Returning NULL for the string when we haven't signaled an error
condition is counter-intuitive and causes unnecessary edge
cases. Return an empty string when asking for a string value for a
configuration variable such as '[section] var' to avoid these edge
cases.

If the distinction between no value and an empty value is needed, this
can be retrieved from the entry directly. As a side-effect, this
change stops the int parsing functions from segfaulting on such a
variable.

Update checkout with new strategies & behavior

'[section] variable' and '[section] variable =' behave differently
when parsed as booleans, so we need to store that distinction
internally.

We're already in the git_config namespace, there is no need to repeat
it.

Win32 CryptoAPI and CNG support for SHA1

Bump the builtin http-parser to v2.0

Fix memory leaks

repository: Refine repository_head() error report

The indexer handles this better than the fetch
logic does.