Seeder code is very old and was still using tools like `getopt()` to
read options. This change attempts to bring the seeder code more in
line with similar files in the code base and modernize the tools it
uses.
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`).
Part of a larger effort to update and clean up seeder code.