Remove some printing code from library

Remove print_lookup_table_entry(), print_dentry(), and
print_security_data() from the library.

Turn wimlib_print_metadata() into a stub and replace it with code in
`wimlib-imagex dir' that uses wimlib_iterate_dir_tree().

Also, on Windows use
ConvertSecurityDescriptorToStringSecurityDescriptorW() to print security
descriptors in string format.