sync manpage with command-line options
In 9fd6c61f, @peterb changed --checkmail to --check-mail. But then be940dc4 was merged, which synced the documentation with the (earlier) implementation.
So this effectively reverts be940dc4, syncing the documentation with the current implementation.
Signed-off-by: Daniel Kahn Gillmor dkg@fifthhorseman.net