Fix orientation error message

When running `scrcpy --capture-orientation=@100`, the error shown is:

> Unsupported orientation: @100 (expected 0, 90, 180, 270, flip0, flip90, flip180 or flip270)

We should tell the user that the error is 100, not @100.

Moreover, the function parse_orientation() should not access the
`optarg` global variable.

PR #6695 <https://github.com/Genymobile/scrcpy/pull/6695>

Signed-off-by: Romain Vimont <rom@rom1v.com>