Before this change, nothing checked that the rc `sync/bisync` call accepted
every bisync flag, so the rc could fall behind the command line as flags
were added, as in #7799.
After this change, `TestRcFlagParity` passes each non-hidden bisync flag to
the rc, under the parameter name `rc.md` uses, with a value of the wrong
type. Every getter `rcBisync` uses rejects it, so the call fails only if
the parameter is read, and the test names any flag the rc doesn't handle.
nielash
be46eaa1f0