split up FS.init

FS.init is mostly responsible for the FS-related initialization of the Module, not the FS.

I've split up the core FS initialization into FS.staticInit.

The point here was removing FS.ensureRoot hack, which doesn't exist in the vfs branch. FS.ensureRoot seems to have existed to handle the core initialization in case you'd disabled the default FS.init.

Ran o1, other and sanity tests.