Optimize 'get-bytevector-some'; it may now read less than possible.

Optimize 'get-bytevector-some'; it may now read less than possible.

* libguile/r6rs-ports.c (scm_get_bytevector_some): Rewrite to
  efficiently take the contents of the read/putback buffers.  In the
  docstring, clarify that it might not return all available bytes.

* doc/ref/api-io.texi (R6RS Binary Input): Clarify that
  'get-bytevector-some' might not return all available bytes.

* test-suite/tests/r6rs-ports.test ("get-bytevector-some [only-some]"):
  Remove bogus test, which requires more than the R6RS requires.