conf, criu: add make_anonymous_mount_file()

conf, criu: add make_anonymous_mount_file()

Before we used tmpfile() to write out mount entries for the container. This
requires a writeable /tmp file system which can be a problem for systems where
this filesystem is not present. This commit switches from tmpfile() to using
the memfd_create() syscall. It allows us to create an anonymous tmpfs file (And
is somewhat similar to mmap().) which is automatically deleted as soon as any
references to it are dropped. In case we detect that syscall is not
implemented, we fallback to using tmpfile().

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>