Minor Bug Fix, Various Improvements

Bug Fix
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Fixed a bug where the DHCP file name would not be properly set because it is based off of defaults set above the parsed arguments. Resolved by using the arguments variables to determine what type of default file name should be used.

Other Improvements
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Moved default settings out of ```__main__``` since it’s pointless to set them there, better usability if they’re set at the very top. ```server.py``` is not meant to be included in another file, that’s why the server modules were split into separate files in an earlier commit (noted in Pull Request #5).

Removed some global variables and simple set the default argument values inline with the code rather than have them in two places and cause confusion.

Reformatted HTTP w/o PXE warning in ```dhcpd.py``` to match that in ```server.py``` for consistency.