Case insensitive checkout improvements

On Mac OS, `realpath` is deficient in determining the actual filename
on-disk as it will simply provide the string you gave it if that file
exists, instead of returning the filename as it exists.  Instead we
must read the directory entries for the parent directory to get the
canonical filename.

When checking out with a case-insensitive working directory, we
want to change the case of items in the working directory to
reflect changes that occured in the checkout target.  Diff now
has an option to break case-changing renames into delete/add.

This reverts commit 40d79154.

Ensure that on a case insensitive filesystem that we can checkout
into some folder 'FOLDER' that exists on disk, even if the target
of the checkout is a different case (eg 'folder').

On Windows, you might sloppily rewrite a file (or have a sloppy
text editor that does it for you) and accidentally change its
case.  (eg, "README" -> "readme").  Git ignores this accidental
case changing rename during checkout and will happily write the
new content to the file despite the name change.  We should, too.

Performance Improvements to Status on Windows

Added call to git_libgit2_shutdown()

Code restructured to better represent best practice when
using libgit2.

Don't search iconv in /opt/local

Since OpenSSL isn't used any more on OS X, there is no dependency
on any MacPorts library under /opt/local and there is no danger of
conflicts between MacPorts and system iconv. For this reason the
system iconv can always be used now.

Using FindFirstFile and FindNextFile in win32 allows us to
use the directory information that is returned, instead of
us having to get the file attributes all over again, which
is a distinct cost savings on win32.

The _next method shouldn't take a path pointer (and a path_len
pointer) as 100% of current users use the full path and ignore
the filename.

Plus let's add some docs and a unit test.

Introduce a new `git_path_diriter` that can iterate directories
efficiently for each platform.

[WIP/RFC] push: report the update plan to the caller

Fix some build warnings

Update documentation for API changes

Fixed unused warning in tests/rebase/merge.c

Added forgotten call to git_libgit2_shutdown() to the /examples/network/git2.c.

Do not call regfree() on an empty regex that is not successfully created...

(also removed an unused member "has_regex" from all_iter)

Changed win32/path_w32.c to utilize NTFS' FindFirst..FindNext data instead of doing an lstat per file. Avoiding unnecessary directory opens and file scans reduces IO, improving overall performance. Effect is magnified due to NTFS being a kernel mode file system (as opposed to user mode).

Minimizing the number directory and file opens, minimizes the amount of IO thus reducing the overall cost of performing ignore operations.

In checkout.c and filter.c we were casting a sub struct
to a parent struct which breaks the strict aliasing rules
in C. However we can use .parent or .base to access the
parent struct to avoid the build warnings.

In remote.c the local variable error was not initialized
or updated in some cases. For unintialized error a build
warning will be generated. So always keep error variable
up-to-date.