truncate: ignore whitespace in --size parameters

truncate: ignore whitespace in --size parameters

Without this, `truncate -s '> -1' F` would truncate F to length 0,
and `truncate -s " +1" F` would truncate F to 1 byte.  Now, each
elicits a diagnostic.
* src/truncate.c: Skip leading white space in the --size option
argument and any white space after one of the relative modifiers,
so that the presence of a +/- modifier can be detected reliably.
* tests/misc/truncate-parameters: Add tests for the above.