Revert "Fix potential string overlap in call to strncat() in TSystem"

This reverts commit 912ee789.

The apparent overlap mentioned in:

  Before reaching the changed line, c is set to be an index into buff,
  so the strings can overlap if no copy is performed, and that leads
  to unpredictable behaviour.

can not happen by construction as the buffer is 'segmented' in 4
independent parts.