String :: toCString - Convert string to C-string

C-strings are strings that could be defined as string constant in a C program. C-strings must not contain several characters, wich are converted as follows:

• \ ==> \\
• newline (10) ==> \n
• carriage return (13) ==> \r
• backspace (8) ==> \b
• form feed (12) ==> \f
• tab (9) == \t
• " ==> \"

Conversion is done inplace. In order to normalize the string later on, fromCString() may be called.