Set up a cancelble context in global options, hook it into the ctrl-C handler
for proper cancel propegation.
Bump up minimal requirement for Go to version 1.7 in documentation
and test-build files.
This defines Linux, FreeBSD and OpenBSD VMs and creates a Go
environment.
Micro-tutorial:
$ vagrant up freebsd
[wait...]
$ vagrant ssh freebsd -c 'cd restic/restic; go test -v ./...'
[test output]
