A Windows manual install has the same directory layout of a normal Unix one
(e.g. the nice symlink node_modules/ep_etherpad-lite -> ../src).
Only when running from the pre-built Windows package the directory layout is
different (e.g. src is physically copied into node_modules/ep_etherpad-lite).
The previous version of the code wrongly assumed that all Windows installs would
be run from the pre-built pakage.
In this version the path search is the same on all platform. If it fails, and we
are on Windows, there is a fallback for the specific case of the pre-built
package.
Fixes#3550
This is just a function (with an ugly side effect for caching purposes) that
heuristically tries to compute the Etherpad installation path when running under
Unix and win32 (they have different file system layouts).
This path can be used by Etherpad as a base for all the relative paths, in order
to be deterministic and not depending on cwd.
