refactor: minor UnixFd cleanups (#376)

* chore: Use std::exchange in UnixFd

This was suggested in code review for #376 .

* fix: Protect against UnixFd self-assignment

While self-assignment is rare, it is expected to be safe.  Add a check
to prevent putting the object in an invalid state.

* fix: Improve hygiene around dup system call

- Don't try to call dup on a negative value.
- Check dup return code and throw if it fails, rather than returning an
  empty UnixFd object.

* chore: Move UnixFd::close to Types.cpp

Minor convenience for applications: unistd.h doesn't have to be included
in the public header.

---------

Co-authored-by: David Reiss <dreiss@meta.com>

src/Types.cpp

@@ -29,6 +29,9 @@
 #include "MessageUtils.h"
 #include SDBUS_HEADER
 #include <cassert>
+#include <cerrno>
+#include <system_error>
+#include <unistd.h>
 
 namespace sdbus {
 
@@ -64,4 +67,27 @@ bool Variant::isEmpty() const
     return msg_.isEmpty();
 }
 
+void UnixFd::close()
+{
+    if (fd_ >= 0)
+    {
+        ::close(fd_);
+    }
+}
+
+int UnixFd::checkedDup(int fd)
+{
+    if (fd < 0)
+    {
+        return fd;
+    }
+
+    int ret = ::dup(fd);
+    if (ret < 0)
+    {
+        throw std::system_error(errno, std::generic_category(), "dup failed");
+    }
+    return ret;
+}
+
 }