Changes how the usernames are loaded.

Allows usernames to be loaded as a file (wordlist), that way the
it's much easier to manage.  It defaults to unix_users.txt,
because these usernames are common in any SSH hosts out there.
If the user only wants to try a specific user (which is better,
because you reduce traffic noise that way), then he/she can set
the USERNAME option, and that should be the only one tried --
similar to how AuthBrute behaves.

I also fixed the regex in check().
Merge branch 'freesshd_authbypass_update' of https://github.com/wchen-r7/metasploit-framework into wchen-r7-freesshd_authbypass_update
msftidy compliant