Add support for checking host against global known host files.

Hi Michael,

I've added support for the standard SSH known hosts files as well as the user known hosts file. Please note there are two possible locations for the global known_hosts file and I check both.

Also, please note I've added a check for existence into the new loop which iterates over the list of hosts files, and if none of the files were found (i.e. the number of files not found is equal to the length of the list), I output the error that you used to output before ("previous known host file not found"). Therefore the original check for existence became obsolete and I've removed it.

Thanks!
iordan