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
diff --git a/libnetdutils/include/netdutils/MemBlock.h b/libnetdutils/include/netdutils/MemBlock.h
index 1b6bd7d..38ca19a 100644
--- a/libnetdutils/include/netdutils/MemBlock.h
+++ b/libnetdutils/include/netdutils/MemBlock.h
@@ -32,11 +32,11 @@
 class MemBlock {
   public:
     MemBlock() : MemBlock(0U) {}
-    MemBlock(size_t len)
+    explicit MemBlock(size_t len)
             : mData((len > 0U) ? new uint8_t[len]{} : nullptr),
               mLen(len) {}
     // Allocate memory of size src.size() and copy src into this MemBlock.
-    MemBlock(Slice src) : MemBlock(src.size()) {
+    explicit MemBlock(Slice src) : MemBlock(src.size()) {
         copy(get(), src);
     }