When calling `git_path_dirname_r` on a Win32 prefix, e.g. a drive
or network share prefix, we always want to return the trailing
'/'. This does not work currently when passing in a path like
'C:', where the '/' would not be appended correctly.

Fix this by appending a '/' if we try to normalize a Win32 prefix
and there is no trailing '/'.

Getting the dirname of a filesystem root should return the filesystem
root itself. E.g. the dirname of "/" is always "/". On Windows, we
emulate this behavior and as such, we should return e.g. "C:/" if
calling dirname on "C:/". But we currently fail to do so and instead
return ".", as we do not check if we actually have a Windows prefix
before stripping off the last directory component.

Fix this by calling out to `win32_prefix_length` immediately after
stripping trailing slashes, returning early if we have a prefix.

This changes the behavior of callbacks so that the callback error
code is not converted into GIT_EUSER and instead we propagate the
return value through to the caller.  Instead of using the
giterr_capture and giterr_restore functions, we now rely on all
functions to pass back the return value from a callback.

To avoid having a return value with no error message, the user
can call the public giterr_set_str or some such function to set
an error message.  There is a new helper 'giterr_set_callback'
that functions can invoke after making a callback which ensures
that some error message was set in case the callback did not set
one.

In places where the sign of the callback return value is
meaningful (e.g. positive to skip, negative to abort), only the
negative values are returned back to the caller, obviously, since
the other values allow for continuing the loop.

The hardest parts of this were in the checkout code where positive
return values were overloaded as meaningful values for checkout.
I fixed this by adding an output parameter to many of the internal
checkout functions and removing the overload.  This added some
code, but it is probably a better implementation.

There is some funkiness in the network code where user provided
callbacks could be returning a positive or a negative value and
we want to rely on that to cancel the loop.  There are still a
couple places where an user error might get turned into GIT_EUSER
there, I think, though none exercised by the tests.

This continues auditing all the places where GIT_EUSER is being
returned and making sure to clear any existing error using the
new giterr_user_cancel helper.  As a result, places that relied
on intercepting GIT_EUSER but having the old error preserved also
needed to be cleaned up to correctly stash and then retrieve the
actual error.

Additionally, as I encountered places where error codes were not
being propagated correctly, I tried to fix them up.  A number of
those fixes are included in the this commit as well.