cmdline: parse numerical options stricter

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