Incremental: accept '--no-degraded' as a deprecated option

Incremental: accept '--no-degraded' as a deprecated option

Commit 3288b419 (Revert "Incremental: honor --no-degraded to delay assembly")
killed the --no-degraded flag since commit 97b4d0e9 (Incremental: honor
an 'enough' flag from external handlers) made this the default behavior
of -I, and brought -I usage for external/container formats in line with
native metadata.  However, this breaks existing usages of '-I
--no-degraded', so allow it as a deprecated option.

Starting a degraded container, like the native metadata case, requires -R.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reported-by: Ignacy Kasperowicz <ignacy.kasperowicz@intel.com>