A new flag `--munge [rename|filter]` controls how a name munging will work.
Name munging only applies to fields named '.', '..', containing a NUL byte, or containing a forward slash '/'.
The `rename` option (the default) will change '.' to '_.' and '..' to '_..'; each NUL byte turns into '_NUL_' and each slash turns into '_SLASH_'.
This greatly simplified policy means that spaces and other special characters should work just fine in field names now.
