Simplify read_string() to just read the characters between the '"'s.

Simplify read_string() to just read the characters between the '"'s.
Push the un-vising up to parse_string(). This allows both the actual
string and the un-vised version to be available as desired. Use
memcpy() instead of strdup() to copy un-vised string since it may
legitimately contain NUL.