date: `iso-8601` argument format discrepancy

The coreutils version currently accepts more argument option-value formats than that of the GNU version:

GNU:
-I[FMT], --iso-8601[=FMT] output date/time in ISO 8601 format...
coreutils:
-I, --iso-8601 [<FMT>] output date/time in ISO 8601 format...

This means the following arguments are accepted by coreutils but not GNU:

• date -I=seconds
• date -I seconds
• date --iso-8601 seconds

GNU will treat =seconds as the value in the first one, and treat seconds as the positional argument [+FORMAT] in the next two.

coreutils will also reject some that are accepted by GNU since it will take the positional argument as the argument value, e.g. date -I 05091009:

error: invalid value '05091009' for '--iso-8601 [<FMT>]'
  [possible values: date, hours, minutes, seconds, ns]

This cannot be simply fixed by setting require_equals for the clap argument, since it would also preclude the correct syntax -Iseconds. There probably are more examples of this discrepancy in coreutils (please comment below).

This is a known limitation in clap, and I have commented in this clap issue and made a fix PR.

Start with the date command's -I/--iso-8601 argument parsing and compare the GNU examples in the issue with clap issue #3030 and PR #5489. Done means the listed argument forms behave like GNU, including preserving the valid -Iseconds syntax.