Allow appending a new detached signature to an existing file

If a new detached signature is created and there is already a file with
the chosen output file name then offer the user to append the new file
to the existing file additionally to the options to overwrite the existing
file or rename the new file.

(cherry picked from commit 39abf52be225bdd38d6ce3659a7511459e53ee30)

• GnuPG-bug-id: T6867