Remove obsolete comment.
Fix memory leak when re-setting variable.
Fix crash on executing unknown command.
Provide error for invalid number in c_set().
New time format: "1h30m7s".

This is fairly flexible, so you could say 10s1h, or even 1h1h1h (which is
equivalent to 3h).  Time parsing is now implemented by a single function, so
there should be no inconsistencies.

The old [[HH:]MM:]SS syntax is still supported, but not used in prompts.