cmdline: parse numerical options stricter

1 - str2offset() no longer accepts negative numbers since offsets are by
nature positive.

2 - introduced str2unum() for the command line parser that accepts
numericals which are not supposed to be negative, so that it will
properly complain on apparent bad uses and mistakes.

Bug: http://curl.haxx.se/mail/archive-2012-07/0013.html

src/tool_helpers.c

@@ -54,6 +54,8 @@ const char *param2text(int res)
     return "is badly used here";
   case PARAM_BAD_NUMERIC:
     return "expected a proper numerical parameter";
+  case PARAM_NEGATIVE_NUMERIC:
+    return "expected a positive numerical parameter";
   case PARAM_LIBCURL_DOESNT_SUPPORT:
     return "the installed libcurl version doesn't support this";
   case PARAM_NO_MEM: