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.

src/truncate.c

@@ -286,6 +286,9 @@ main (int argc, char **argv)
           break;
 
         case 's':
+          /* skip any whitespace */
+          while (isspace (*optarg))
+            optarg++;
           switch (*optarg)
             {
             case '<':
@@ -305,6 +308,9 @@ main (int argc, char **argv)
               optarg++;
               break;
             }
+          /* skip any whitespace */
+          while (isspace (*optarg))
+            optarg++;
           if (*optarg == '+' || *optarg == '-')
             {
               if (rel_mode)