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.

Fix wrong format string in git_reflog_drop() error message

Validate configuration keys

Use SecureTransport on OS X

On close, we might get a return code which looks like an error but just
means that the other side closed gracefully. Handle that.

Anything SSL is deprecated. Let's make sure we don't try to use SSL v3
when talking to the server.

Do not automatically fail on a bad certificate, but let the caller
decide. This means we don't need our switch on errors anymore but can
return a string representation from the security framework.

This is what it's meant all along, but now we actually have multiple
implementations, it's clearer to use the name of the library.