src/tool_paramhlp: try harder to catch negatives

strto* functions happily chomp off leading whitespace, so simply
checking for str[0] can lead to false negatives. Do the full parse and
check the out value instead.

src/tool_paramhlp.c

@@ -178,9 +178,13 @@ ParameterError str2num(long *val, const char *str)
 
 ParameterError str2unum(long *val, const char *str)
 {
-  if(str[0]=='-')
-    return PARAM_NEGATIVE_NUMERIC; /* badness */
-  return str2num(val, str);
+  ParameterError result = str2num(val, str);
+  if(result != PARAM_OK)
+    return result;
+  if(*val < 0)
+    return PARAM_NEGATIVE_NUMERIC;
+
+  return PARAM_OK;
 }
 
 /*