Taught the assistant to stop reusing an existing `git annex transferkeys`
process after it detects a network connection change. I don't think this is
a complete solution to what to do about long-duration network connections
in remotes. For one thing a remote could take a long time to time out
when the network is disconnected, and block other transfers (eg to
local drives) in the meantime. But at least if a remote loses its network
connection and does not try to reconnect on its own, and so is continually
failing, this will get it back into a working state eventually.

Also, fixed a problem with the OSX Mavericks build, it seems that the
versions of wget and coreutils stuff that I was including in it were built
by homebrew with full optimisations turned on, so didn't work on some CPUs.
Replaced those with portable builds.
