Support for getcwd and chdir system calls

[cahirwpz]

• Store vnode to current working directory somewhere in proc structure:
  • FreeBSD keeps it in p_fd that contains filedesc.fd_cdir,
  • NetBSD in p_cwdi which contains cwdinfo.cwdi_cdir.
• Improvements to vfs_lookup:
  • support relative path lookups with .. and . at the beginning or in the middle of a path,
  • normalize path by removing consecutive /, ./, etc.

[cahirwpz]

Already done!