Have string ports honor `%default-port-conversion-strategy'.

Have string ports honor `%default-port-conversion-strategy'.

* libguile/strports.c (scm_mkstrport): Remove initialization of
  `pt->ilseq_handler'.

* module/ice-9/pretty-print.scm (truncated-print)[ellipsis]: Set
  %DEFAULT-PORT-CONVERSION-STRATEGY to 'error.

* test-suite/tests/ports.test ("string
  ports")["%default-port-conversion-strategy is honored"]: New test.
  ["wrong encoding"]: Rename to...
  ["wrong encoding, error"]: ... this.  Explicitly set
  %DEFAULT-PORT-CONVERSION-STRATEGY to 'error.  Return #f when no
  exception is raised.