Fix unicode module declaration (copy-paste error)
unicode.lua: Add some UTF-8 decoding error checking
Add CP437 (native Windows OEM) to unicode.lua
Add extended-ASCII support to LM hashing

Probably only works for US-English, since it uses Code Page 437. Adding
support for other locales would require detecting or setting the locale
for the remote system, since SMB has no way to negotiate code page. In
other words, Windows systems with different locales will have different
LM hashes for the same password.

Also added some tests. Hashes confirmed by googling for them and finding
the correct plaintext.
Fix NSEdoc formatting in unittest.lua
Add unittest.testing() to make test building conditional