adb: shell: add -n flag to not read from stdin.

Shell scripts of the following form do not work properly with adb:
    echo "foo\nbar\nbaz" | {
        read FOO
        while [ "$FOO" != "" ]; do
            adb shell echo $FOO
            read FOO
        done
    }
The first run of adb shell will consume all of the contents of stdin,
causing the loop to immediately end. ssh solves this by providing a -n
flag that causes it to not read from stdin. This commit adds the same.

Bug: http://b/25817224
Change-Id: Id74ca62ef520bcf03678b50f4bf203916fd81038
diff --git a/adb/adb_utils.cpp b/adb/adb_utils.cpp
index 42f1c7d..1c3526b 100644
--- a/adb/adb_utils.cpp
+++ b/adb/adb_utils.cpp
@@ -30,12 +30,31 @@
 #include <base/stringprintf.h>
 #include <base/strings.h>
 
+#include "adb.h"
 #include "adb_trace.h"
 #include "sysdeps.h"
 
 ADB_MUTEX_DEFINE(basename_lock);
 ADB_MUTEX_DEFINE(dirname_lock);
 
+#if defined(_WIN32)
+constexpr char kNullFileName[] = "NUL";
+#else
+constexpr char kNullFileName[] = "/dev/null";
+#endif
+
+void close_stdin() {
+    int fd = unix_open(kNullFileName, O_RDONLY);
+    if (fd == -1) {
+        fatal_errno("failed to open %s", kNullFileName);
+    }
+
+    if (TEMP_FAILURE_RETRY(dup2(fd, STDIN_FILENO)) == -1) {
+        fatal_errno("failed to redirect stdin to %s", kNullFileName);
+    }
+    unix_close(fd);
+}
+
 bool getcwd(std::string* s) {
   char* cwd = getcwd(nullptr, 0);
   if (cwd != nullptr) *s = cwd;