NetworkManager is now responsible for creating and
and there is no way to customize
what it writes; it only writes exactly what the dhcp server says. What it writes there can be modified by right-clicking the NetworkManager tray icon and editing the connections. Thanks, Adrian, for the updated info.
resolv.conf(5) says that you can set LOCALDOMAIN environment variable to amend the search option in resolv.conf, but that doesn't actually work for all programs. ssh works, ping doesn't, for example.