Cleaned up strings::tokenize.

Trivial cleanup of Stout's tokenize function.

Also, this is last change I needed to get Clang Static Analyzer to build and analyze Mesos on Ubuntu 13.04.
(It was complaining about offset never being read after offset = s.length();).

From: David Mackey <tdmackey@booleanhaiku.com>
Review: https://reviews.apache.org/r/14288
Added the ability to parse Byte flags.

Review: https://reviews.apache.org/r/14042
Fixed io_test to expect a successful return value from pipe().

Verify that pipe() did not fail before trying to use the resulting pipe.

Signed-off-by: David Mackey <tdmackey@booleanhaiku.com>

From: David Mackey <tdmackey@booleanhaiku.com>
Review: https://reviews.apache.org/r/14245
Removed duplicated definition of gethostbyname2_r.

When compiling on an Apple the gethostbyname2_r function is defined
twice; once in libprocess/src/pid.cpp and once in stout/os.hpp.
This patch removes the unnecessary definition from
libprocess/src/pid.cpp.

From: David Mackey <tdmackey@booleanhaiku.com>
Review: https://reviews.apache.org/r/13988