GitLab

Detail:
  Remove undocumented 't' mode which forcibly timestamped files, since other operating systems use this to denote a text file (as opposed to 'b' binary mode).
  Add 'x' mode per ISO9899:2018
  * only applies to 'w' mode files (not 'r' or 'a')
  * file must not exist prior to creation
  * beyond that we rely on RISC OS' policy of only allowing a file to be opened once for writing as our exclusivity
  Also fix bug in check for 'Protected disc' error, which only trigged for fsnumber_ADFS (8).
Admin:
  Tested in a ROM build with a simple test harness.