Grab bag of netdutils fixes

Includes:
    [1] make MemBlock's constructors explicit
    [2] fix "unused variable" problems in FdTest.
    [3] fix std::string output for StatusOr
        (value printing was never implemented)
    [4] add expectation in SyscallsTest for close because of UniqueFd

Test: as follows
    - built, flashed, booted
    - system/net/tests/runtests.sh passes
    - make netdutils_test && adb push out/target/product/<deviced>/data/nativetest64/netdutils_test/netdutils_test /data/nativetest64/netdutils_test && adb shell /data/nativetest64/netdutils_test
Change-Id: I0f6e7b8544d4a88e73e50600de28eed34ef6cc2d