kernel_cmdline: Improve parameter parsing with validation and whitespace handling

- Convert from infallible From traits to fallible TryFrom for parameter parsing
- Add validation to reject keys with internal whitespace
- Improve whitespace handling by trimming leading/trailing ascii whitespace
- Filter out empty chunks from extra whitespace between parameters
- Add comprehensive test coverage for new validation and edge cases
- Maintain backward compatibility for valid kernel command line formats

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Signed-off-by: John Eckersberg <jeckersb@redhat.com>

Author: jeckersb