Silence GCC warning.

* libguile/fports.c (scm_setvbuf): Initialize 'drained'.  GCC 4.9 raises
  a "may be used uninitialized warning" for 'drained', because it can't
  tell that 'drained' is initialized anytime NDRAINED > 0.
tests: Add test for _IOLBF.

* test-suite/tests/ports.test ("pipe, fdopen, and _IOLBF"): New test.
web: Keep the default size for the client's in-kernel receive buffer.

Fixes <http://bugs.gnu.org/15368>.

* module/web/client.scm (open-socket-for-uri): Remove call to
  'setsockopt'.  Contrary to what the comment said, its effect was to
  shrink the receive buffer from 124 KiB (the default size, per
  /proc/sys/net/core/rmem_default on Linux-based systems) to 12 KiB.