Disallow operator!() on unique_fd

This catches a common mistake where client code checks for errors using
the common idiom that works for std::iostream and other file-like
classes:

  unique_fd fd = open(...);
  if (!fd) {
  }

Test: atest libbase_test
Test: m droid
Change-Id: I9629a7795537ecb3b57be9c741c06f80967e4cc2

base/include/android-base/unique_fd.h

@@ -105,6 +105,9 @@ class unique_fd_impl final {
   int get() const { return fd_; }
   operator int() const { return get(); }  // NOLINT
 
+  // Catch bogus error checks (i.e.: "!fd" instead of "fd != -1").
+  bool operator!() const = delete;
+
   int release() __attribute__((warn_unused_result)) {
     tag(fd_, this, nullptr);
     int ret = fd_;

init/service.cpp

@@ -177,7 +177,7 @@ Result<Success> Service::SetUpPidNamespace() const {
 Result<Success> Service::EnterNamespaces() const {
     for (const auto& [nstype, path] : namespaces_to_enter_) {
         auto fd = unique_fd{open(path.c_str(), O_RDONLY | O_CLOEXEC)};
-        if (!fd) {
+        if (fd == -1) {
             return ErrnoError() << "Could not open namespace at " << path;
         }
         if (setns(fd, nstype) == -1) {