Rewrite get_iconv_codepoint to fix a bug involving byte-order marks.

Rewrite get_iconv_codepoint to fix a bug involving byte-order marks.

* libguile/ports.c (get_iconv_codepoint): Rewrite to fix a bug and
  improve efficiency and clarity.  Previously, it incorrectly assumed
  that iconv would never consume input without producing output, which
  led to a buffer overrun and subsequent assertion failure.  This
  happens when a byte-order mark is consumed by iconv at the beginning
  of the stream when using the UTF-16 or UTF-32 encodings.

* test-suite/tests/ports.test (unicode byte-order marks (BOMs)):
  Add tests.