daemon: Introduce "pulse mode" progress events.

daemon: Introduce "pulse mode" progress events.

This introduces a new form of progress event, where we don't know how
much of the operation has taken place, but we nevertheless want to
send back some indication of activity.  Some progress bar indicators
directly support this, eg. GtkProgressBar where it is known as "pulse
mode".

A pulse mode progress message is a special backwards-compatible form
of the ordinary progress message.  No change is required in callers,
unless they want to add support for pulse mode.

The daemon sends:

 - zero or more progress messages with position = 0, total = 1
 - a single final progress message with position = total = 1

Note that the final progress message may not be sent if the call fails
and returns an error.  This is consistent with the behaviour of
ordinary progress messages.

The daemon allows two types of implementation.  Either you can just
call notify_progress (0, 1); ...; notify_progress (1, 1) as usual.

Or you can call the functions pulse_mode_start, pulse_mode_end and/or
pulse_mode_cancel (see documentation in daemon/daemon.h).  For this
second form of call, the guarantee is very weak: it *just* says the
daemon is still capable of doing something, and it doesn't imply that
if there is a subprocess that it is doing anything.  However this does
make it very easy to add pulse mode progress messages to all sorts of
existing calls that depend on long-running external commands.

To do: add a third variant that monitors a subprocess and only sends
back progress messages if it's doing something, where "doing
something" might indicate it's using CPU time or it's printing output.