PR 1706 plus error reporting cleanups

The SSH error checking and reporting could still be further
improved by using the libssh2 native methods to get error info,
but at least this ensures that all error codes are checked and
translated into libgit2 error messages.

Conflicts:
	src/transports/ssh.c

Remove GIT_STATUS_SHOW_INDEX_THEN_WORKDIR option

API should not be ifdeffed

Fix git_config_get_multivar with interleaved missing entries

The old tests didn't try failing lookups or lookups across
multiple config files with some having the pattern and some
not having it.

If there is not an error, the return value was always the return value
of the last call to file->get_multivar

With this commit GIT_ENOTFOUND is only returned if all the calls to
filge-get_multivar return GIT_ENOTFOUND.

This also adds a test that actually calls git_libgit2_capabilities
and git_libgit2_version.

This makes all of the credential objects use the same pattern to
clear the contents and call git__memzero when done.  Much of this
information is probably not sensitive, but it also seems better
to just clear consistently.

Null byte in hunk header

Much of the SSH credential creation API can be left enabled even
on platforms with no SSH support.  We really just have to give an
error when you attempt to open the SSH connection.

The SSH APIs will just return an error code and state that the
library was built without SSH support if they are called in
that case.

Make the git_signature `const` in the stash API.

The diff hunk context string that is returned to xdiff need not
be NUL terminated because the xdiff code just copies the number of
bytes that you report directly into the output.  There was an off
by one in the diff driver code when the header context was longer
than the output buffer size, the output buffer length included
the NUL byte which was copied into the hunk header.

Fixes #1710

Segmentation fault on git_clone

Fixes #1700.

This option serves no benefit now that the git_status_list API
is available.  It was of questionable value before and now it
would just be a bad idea to use it rather than the indexed API.

Fix segfault in git_status_foreach_ext()

Add tests for the `GIT_STATUS_SHOW_XXX` flags.

[trivial] Fix small typo in docs for git_repository_message.

[trivial] Fix docs to use proper enum names

Fix Win32/64 compilation warnings & memory leak

Test submodules with empty index or orphaned head

In both of these cases, the submodule data should still be loaded
just (obviously) without the data that comes from either the index
or the HEAD.

This fixes a bug in the orphaned head case.

Dirty submodules do not correctly appear in diffs

There was a bug where submodules whose HEAD had not been moved
were being marked as having an UNMODIFIED delta record instead
of being left MODIFIED.  This fixes that and fixes the tests to
notice if a submodule has been incorrectly marked as UNMODIFIED.