More mingw32 compilation fixes.

The fix to support attrs on bare repos went a little too far
in trying to avoid using the working directory and ended up
not processing the input path quite correctly.

compat: make p_open able to accept optional mode when passing the O_CREAT flag

This has the nice side effect of making test_attr_repo__staging_properly_normalizes_line_endings_according_to_gitattributes_directives() test pass again on Windows. This test started to fail after commit 674a1985 was applied.

It can cause segfaults if the call didn't set an error

Previously, it was defined in netops.c, but it's also needed in one of the
clar tests, so I figured we might as well just make it global for the
whole project.

Without it, the mingw32 linker won't resolve GetProcessId() (called from
the core/errors.c clar test) because of some conditionals in windows.h.

Fixes the mingw32 build issues.

Conflicts:
	src/netops.c
	src/netops.h
	src/transports/http.c
	tests-clar/clar

New-error-handling

Support git attrs from index (and bare repo)

Honor core.notesRef config option

gitno_connect() can return an error or socket, which is fine on most
platforms where sockets are file descriptors (signed int), but on Windows,
SOCKET is an unsigned type, which is problematic when we are trying to
test if the socket was actually a negative error code.

This fix seperates the error code and socket in gitno_connect(), and fixes
the error handling in do_connect() functions to compensate. It appears
that git_connect() and the git-transport do_connect() functions had bugs
in the non-windows cases too (leaking sockets, and not properly reporting
connection error, respectively) so I went ahead and fixed those too.

MINGW32 does not define _mktemp_s, so we can just use _mktemp instead. See
the non-compressed/non-base64-encoded version of the patch here:
http://gist.github.com/2605249

There are three changes here:
- correctly propogate error code from failed object lookups
- make zlib inflate use our allocators
- add OID to notfound error in ODB lookups

Depending on the operation, we need to consider gitattributes
in both the work dir and the index.  This adds a parameter to
all of the gitattributes related functions that allows user
control of attribute reading behavior (i.e. prefer workdir,
prefer index, only use index).

This fix also covers allowing us to check attributes (and
hence do diff and status) on bare repositories.

This was a somewhat larger change that I hoped because it had
to change the cache key used for gitattributes files.

Add git_note_default_ref to allow easy retrieval of the currently set
default notes reference.

Setting core.notesRef allows to change the default notes reference used
by Git. Check if set before using GIT_NOTES_DEFAULT_REF. Fixes #649.

Conflicts:
	.travis.yml
	include/git2/diff.h
	src/config_file.c
	src/diff.c
	src/diff_output.c
	src/mwindow.c
	src/path.c
	tests-clar/clar_helpers.c
	tests-clar/object/tree/frompath.c
	tests/t00-core.c
	tests/t03-objwrite.c
	tests/t08-tag.c
	tests/t10-refs.c
	tests/t12-repo.c
	tests/t18-status.c
	tests/test_helpers.c
	tests/test_main.c

Since strnlen is not supported on all platforms and since we
now have the shiny new git_text_is_binary in the filtering
code, let's convert diff binary detection to use the new stuff.

To make this code more resilient to future changes, we'll
explicitly translate the libgit2 structure to the libxdiff
structure.