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`).
This patch also integrates chainparams with the seeder.
Release notes have been added and the Seeder's `README` file have
been edited to reflect this these changes.
Depends on D4417