Implement #!fold-case and #!no-fold-case reader directives.

Implement #!fold-case and #!no-fold-case reader directives.

* libguile/read.c (set_port_case_insensitive_p): New function.

  (scm_read_shebang): Handle #!fold-case and #!no-fold-case.

* doc/ref/api-evaluation.texi (Case Sensitivity, Scheme Read): Document
  the #!fold-case and #!no-fold-case reader directives.

* test-suite/tests/reader.test ("per-port-read-options"): Add tests.