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 (like bitcoind.cpp) in the code base and modernize the tools it
uses. To accomplish this, the CDnsSeedOpts class has been removed as its purpose
is made almost completely obsolete by ArgsManager.
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).
Depends on D4222