Make socket I/O non-blocking and add optional timeouts.

Users may now set `GIT_OPT_SET_SERVER_CONNECT_TIMEOUT` to set a shorter
connection timeout. (The connect timeout cannot be longer than the
operating system default.) Users may also now configure the socket read
and write timeouts with `GIT_OPT_SET_SERVER_TIMEOUT`.

By default, connects still timeout based on the operating system
defaults (typically 75 seconds) and socket read and writes block.

Add a test against our custom testing git server that ensures that we
can timeout reads against a slow server.

v0.6.0 of poxygit add support for throttling connections to test
timeouts and low-bandwidth situations.

SHA256: more SHA256 support

The git included with xenial is ancient, and lacks sha256 support.

We provide `BUILD_PATH` to our build script; provide it and mutate
`PATH` when running our tests as well.

We were previously using `cygpath` to try to convert a _list_ of Windows
paths into cygwin / Unix style `PATH` format. This does not work -- it
treats the path list as a single path (with semicolons -- understandably
as those are allowed characters in a Windows path).

For example, `C:\One;C:\Two;C:\Three` is converted to
`/c/one;c:/two;c:/three`.

Add a new function to convert path lists, so that paths are split by
semicolon and fed to `cygpath` independently, then re-joined with a
colon. This means that our example `C:\One;C:\Two;C:\Three` is correctly
converted to `/c/one:/c/two:/c/three`.

We provide `BUILD_PATH` to our build script; provide it and mutate
`PATH` when running our tests as well.

libgit2 can now isolate its home directory, and our test runner (by
default) isolates the home directory. In our CI environment, we want to
set up some pieces (like ssh configuration) in a fake homedir. Continue
to do so and propagate that to clar.