Fix babysitter script to work on Windows (Cygwin) builders

Fix babysitter script to work on Windows (Cygwin) builders

The babysitter script used for CI results was previously able to run
either on Windows or standard UNIX. However, the builders run Cygwin,
which turns out to need special support. To make Cygwin work two
changes were made:

1. Paths which are exported to Mono get filtered through `cygpath`.
Paths received back from Mono are filtered in reverse. The script now
tracks two sets of path strings.

2. Under certain circumstances in Cygwin, pgkill() attempts to close a
process group and finds it does not exist. My assumption is that this
is because it has already been closed. I dealt with this by simply
catching the failure and printing a warning.