string_interpret_escape in string.c does not work with escapes for " (\")
and \ (\\).
This is a problem, if someone uses something like this:
${extract {field} {field="Value \" with \\ strange signs"}}
i know this could be escaped with \xx where xx is the value of the char,
but i think escaping \ and " with \ is very common.
one way (maybe not the cleanest one) would be to place the ch=0 inside of
the isxdigit-check and not before it, this would make everything thats
no specialy processed return directly.
