tool: New option --data-raw to HTTP POST data, '@' allowed.

Add new option --data-raw which is almost the same as --data but does
not have a special interpretation of the @ character.

Prior to this change there was no (easy) way to pass the @ character as
the first character in POST data without it being interpreted as a
special character.

Bug: https://github.com/bagder/curl/issues/198
Reported-by: Jens Rantil