refactor path resolution

- Better path resolution. WASI applications should have
  no knowledge of the underlying host file system. Prior
  to this commit, those details were exposed in a few
  edge cases.
- Added support for reading and following symlinks.
  Prior to this commit, that worked, but the content of
  the symlinks were based on the host syste, which was
  wrong.
- Writing symlinks works on non-Windows platforms now.
- Remove use of strtok().