Previously the only way to remove an option from a remote was to set it
to an empty string, which is not the same as deleting it - a present but
empty value overrides the option's default whereas a deleted key
restores it. Editing the file by hand isn't an option for an encrypted
config either.
This adds a "config unset" command and a "config/unset" rc endpoint to
remove one or more keys from an existing remote.