Replaces getopts() with gArgs.
Because ArgsManager does not seem to support both long and short
options and because it also reserves the -h option for help message
requests, the seeder's options have all been changed to long-form. This
is also more inline with our other code (for example, see bitcoind.cpp).
Release notes have been added and the Seeder's README file have
been edited to reflect this these changes.