test, add regression cases for non-existent input/output files
Merge branch 'test/regressions'

* test/regressions:
  test, add regression cases for non-existent input/output files
fix, check if output initialisation fails in init() of ute-shnot(1)
hygiene, don't call exit(1) outside of main()
hygiene, constify read-only slots in shnot_ctx_s struct
minor, count the number of successful input file shnot runs and delete outfile if none was succeeded
test, adapt regression for ute shnot(1) to match new return code policy
Merge branch 'cmd/shnot'

* cmd/shnot:
  test, adapt regression for ute shnot(1) to match new return code policy
  minor, count the number of successful input file shnot runs and delete outfile if none was succeeded
  hygiene, constify read-only slots in shnot_ctx_s struct
  hygiene, don't call exit(1) outside of main()
  fix, check if output initialisation fails in init() of ute-shnot(1)
hygiene, constify read-only slots in chndl_ctx_s struct
fix, check if output initialisation fails in init() of ute-chndl(1)
minor, count the number of successful input file chndl runs and delete outfile if none succeeded
test, adapt regression for ute chndl(1) to match new return code policy
Merge branch 'cmd/chndl'

* cmd/chndl:
  test, adapt regression for ute chndl(1) to match new return code policy
  minor, count the number of successful input file chndl runs and delete outfile if none succeeded
  fix, check if output initialisation fails in init() of ute-chndl(1)
  hygiene, constify read-only slots in chndl_ctx_s struct
hygiene, constify read-only slots in pr_ctx_s struct
chore, use error() to output errors in ute print(1)
minor, count the number of successful input file print runs and delete outfile if none was succeeded
test, adapt regression for ute print(1) to match new return code policy
Merge branch 'cmd/print'

* cmd/print:
  test, adapt regression for ute print(1) to match new return code policy
  minor, count the number of successful input file print runs and delete outfile if none was succeeded
  chore, use error() to output errors in ute print(1)
  hygiene, constify read-only slots in pr_ctx_s struct
minor, count the number of successful input file mux runs and delete outfile if none was succeeded
test, adapt regression for ute mux(1) to match new return code policy