dhcp-option: refuse control and non-UTF8 characters in string option

dhcp-option: refuse control and non-UTF8 characters in string option

We oftem save parsed DHCP options into a file, or expose them
through DBus or Varlink. In such case, control characters or non-UTF8
characters may cause many kind of unexpected errors. In general, a DHCP
message that have string options with spurious characters is mostly
malformed or broken. Let's refuse them.

This also makes dhcp_option_parse_string() do not free 'ret' argument,
to follow our usual coding style. So, callers now need to free the
pre-exisitng string if necessary.

Fixes #31708.