Offer appending new detached signature only to existing detached signatures

If the existing file is a detached OpenPGP signature then allow appending
the new detached signature to the existing signature(s). Otherwise, only
allow to overwrite the existing file or change the name the new file.

(cherry picked from commit 46fda9a531fe01ab928867be6d05de9fd8e14eaa)

• GnuPG-bug-id: T6867