tool: avoid generating ambiguous escaped characters in --libcurl

tool: avoid generating ambiguous escaped characters in --libcurl

C string hexadecimal-escaped characters may have more than 2 digits.
This results in a wrong C compiler interpretation of a 2-digit escaped
character when followed by an hex digit character.

The solution retained here is to represent such characters as 3-digit
octal escapes.

Adjust and extend test 1465 for this case.

Closes #9643