Update mo_os

mo_os

• add change_dir to have a compiler independent chdir routine
• add get_cwd to have a compiler independent getcwd routine
• add path constants curdir, pardir, sep, extsep, linesep and devnull
• path_splitext:
  • don't check for folder
  • ignore leading dots in tail of path
  • make root and ext optional
• path_split:
  • remove trailing '/' from head unleass it is root (e.g. '/' or '//' or '///' and so on)
  • make head and tail optional
• added path_dirname and path_basename (path_split's head and tail)
• added path_root and path_ext (path_splitext's root and ext)
• added path_stem (final path component without suffix)
• converted path_[exists|isfile|isdir] to lowlevel functions
  • original subroutines are now called check_path_[exists|isfile|isdir]
• added path_isabs and path_isroot checking functions
• added path_normpath to normalize path
• added path_abspath to get absolute path
• added path_parts subroutine to split path into all components
• added path_as_posix function to replace \\ with / in given path

mo_string_utils

• added strip argument to startswith
• add endswith function