Minor Bug Fix, Various Improvements

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.

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.

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 math that in server.py
for consistency.