Adding an agent with a store credential on an instance with no
<configDir>/secrets/agents left the bus unable to start
(credential-file): Credentials refuses a missing store root, and only
issue() made it, which needs a running bus. Start now makes the root
(0700) the same way issue() does, only when some reference has
store: true. Credentials' own refusal (credential-store T1) is unchanged,
and a root with loose modes or a symlink still refuses start.
Tests: two new credential-verbs tests; the creation test fails on
483eccb3d with credential-file. All 171 bus test files run one at a
time: 1713 pass, 0 fail.