libcamera: base: file_descriptor: Return UniqueFD from dup()

The dup() function returns a duplicate of the file descriptor. Wrapping
it in a FileDescriptor isn't wrong as such, but it prevents from using
it in contexts where a UniqueFD is needed. As the duplicate is
guaranteed to have a single owner when created, return it as a UniqueFD
instead. A FileDescriptor can easily be created from the UniqueFD if
desired.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

include/libcamera/base/file_descriptor.h

@@ -28,7 +28,7 @@ public:
 
 	bool isValid() const { return fd_ != nullptr; }
 	int fd() const { return fd_ ? fd_->fd() : -1; }
-	FileDescriptor dup() const;
+	UniqueFD dup() const;
 
 private:
 	class Descriptor