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_paramhlp.h

@@ -32,6 +32,7 @@ ParameterError file2memory(char **bufp, size_t *size, FILE *file);
 void cleanarg(char *str);
 
 int str2num(long *val, const char *str);
+int str2unum(long *val, const char *str); /* for unsigned input numbers */
 
 long proto2num(struct Configurable *config, long *val, const char *str);