warn about filename characters unsupported on other platforms

Once lp:~jelmer/brz/support-backslash lands, it will be possible to add fileanmes with backslashes in the name on POSIX platforms.

However, it will still not be possible to check out filenames with backslashes on Windows. Because of this, it would be nice if "bzr add" would either just warn or perhaps require --force to be specified to add filenames with problematic characters ("\", "<", ">", etc) on other platforms.