Quote arguments to logging functions

The user-visible difference is that whitespace is not collapsed in
logging output.
Align output of snapshot age report
Don't print trailing '0s' for durations

If the argument to 'duration' is 0, then print '0s' so as to avoid
printing nothing at all, but otherwise if the seconds value is 0, then
skip it.
Add logging trace for duration function
Add progress display for initial send

Refactor progress display and add it to the initial send process. Also,
display (with '-v') the estimated send size.
Remove dead code

This old version of expire_src_snaps has been commented out since
approximately forever.
Improve output of age reporting in error cases

The existence of the source and destination pools is tested before
running the check, as otherwise specifying an invalid pool would cause
no results to be printed, as if everything is up to date.

Also, when '-v' is specified, be explicit about what pool is being
checked, and display a notification if there appears to be a
synchronisation currently in progress to the given destination.