More Windows porting.. Seem to be getting near an end of the easy stuff,
and also the webapp is getting pretty usable on Windows now, the only
really important thing lacking is XMPP support.

Made git-annex on Windows set HOME when it's not already set. Several of
the bundled cygwin tools only look at HOME. This was made a lot harder and
uglier due to there not being any way to modify the environment of the
running process.. git-annex has to re-run itself with the fixed
environment.

Got rsync.net working in the webapp. Although with an extra rsync.net
password prompt on Windows, which I cannot find a way to avoid.

While testing that, I discovered that openssh 6.5p1 has broken support for
~/.ssh/config Host lines that contain upper case letters! I have filed a
bug about this and put a quick fix in git-annex, which sometimes generated
such lines.