Fix get_option_value() for strings with spaces

Fix get_option_value() for strings with spaces

Problem is that

istream& operator>> (istream& is, char* str );

according to C++ documentation "Ends extraction when the
next character is either a valid whitespace or a null character,
or if the End-Of-File is reached."

So if the parameter value is a string with spaces the currently
used instruction 'ss >> ret;' copies the chars only up to the first
white space and not the whole string.

Use a specialization of get_option_value() to fix this corner case.

Bug reported by xiaozhi

Signed-off-by: Marco Costalba <mcostalba@gmail.com>