Revise tempfile system

[nobodyinperson]

Some commands don't need a tempfile for caching the output (e.g. if they don't overwrite the oroginal file but create a new file or if the output file would be very large).

The m4 macros should be reorganised to allow not using a tenpfile in a flexible way.

Proposal:

• Introduce TUCA_CMD
  • which is wrapped around the whole command
  • does basic initialisation like TUCA_INIT
  • takes the command as argument
  • takes another argument to switch tempfile usage on/off
    • defining a TUCA_LOOP_FILETMP_VAR if a tempfile is desired
    • other progress-indicating macros like TUCA_LOOP and TUCA_WAIT should then use the tempfile accordingly or not