Turn --sign=BOOL into --sign=no|best-effort|require-one|require-all,
making the multi-signer aggregation policy explicit: best-effort never
fails on signing, require-one requires at least one signature, and
require-all requires every signer to succeed (an empty reply, i.e. a
signer opting out, counts as failure). Signed reports are always emitted
as a JSON-SEQ stream. The mode is also exposed as an input to the
io.systemd.Report.GenerateSigned Varlink method.
Signed-off-by: Paul Meyer <katexochen0@gmail.com>