Clean up rustc warnings.

compiletest: compact "linux" "macos" etc.as "unix".
liballoc: remove a superfluous "use".
libcollections: remove invocations of deprecated methods in favor of
    their suggested replacements and use "_" for a loop counter.
libcoretest: remove invocations of deprecated methods;  also add
    "allow(deprecated)" for testing a deprecated method itself.
libglob: use "cfg_attr".
libgraphviz: add a test for one of data constructors.
libgreen: remove a superfluous "use".
libnum: "allow(type_overflow)" for type cast into u8 in a test code.
librustc: names of static variables should be in upper case.
libserialize: v[i] instead of get().
libstd/ascii: to_lowercase() instead of to_lower().
libstd/bitflags: modify AnotherSetOfFlags to use i8 as its backend.
    It will serve better for testing various aspects of bitflags!.
libstd/collections: "allow(deprecated)" for testing a deprecated
    method itself.
libstd/io: remove invocations of deprecated methods and superfluous "use".
    Also add #[test] where it was missing.
libstd/num: introduce a helper function to effectively remove
    invocations of a deprecated method.
libstd/path and rand: remove invocations of deprecated methods and
    superfluous "use".
libstd/task and libsync/comm: "allow(deprecated)" for testing
    a deprecated method itself.
libsync/deque: remove superfluous "unsafe".
libsync/mutex and once: names of static variables should be in upper case.
libterm: introduce a helper function to effectively remove
    invocations of a deprecated method.

We still see a few warnings about using obsoleted native::task::spawn()
in the test modules for libsync.  I'm not sure how I should replace them
with std::task::TaksBuilder and native::task::NativeTaskBuilder
(dependency to libstd?)

Signed-off-by: NODA, Kai <nodakai@gmail.com>
auto merge of #17970 : nodakai/rust/cleanup-warnings, r=alexcrichton

These are basically all straightforward changes and should not impact behavior of the language.