Detecting cygwin via PATH contents can result in false positives
resulting in errors when using Vagrant on Windows outside of a
cygwin shell. Use environment based detection instead.
While VirtualBox has commented that they do not support UNC remote
paths (but do for long paths) it seems that remote paths can work.
If user provides UNC path, allow it to be used as-is.
Fixes#7011
This commit changes Vagrant::Util::Platform to cache the result of some
common operations. These values are highly unlikely to change over the
course of a single Vagrant run and they are only cached for that run.
