Fix `setvbuf' to leave the line/column number unchanged.

Fix `setvbuf' to leave the line/column number unchanged.

* libguile/fports.c (scm_setvbuf): Use `scm_take_from_input_buffers'
  directly instead of `scm_drain_input'; use `scm_unget_byte' instead of
  `scm_unread_string' to put the drained input back to PORT.  This
  leaves PORT's line/column numbers unchanged, whereas they'd previously
  be decreased by the `scm_unread_string' call.

* libguile/ports.c (scm_take_from_input_buffers): Update description and
  variable names to refer to "bytes", not "chars".

* test-suite/tests/ports.test ("setvbuf"): New test prefix.