handle non-integer flags

From https://github.com/derhuerst/node-sockopt/blob/19410b3a048efa3bbb3f955e677e438bbdc6104d/index.js#L21-L25:

> Currently `_setsockopt` (& `_getsockopt`) only handles integers.
> todo: fails with e.g. `SO_RCVTIMEO`/`SO_SNDTIMEO`
>
> ```js
> if (!Number.isInteger(flagValue)) {
> throw new Error('flagValue must be an integer')
> }
> ```

The native code would have to be adapted to create the appropriate `struct`s for selected flags.

Start with index.js lines 21-25 and trace the native implementations of _setsockopt and _getsockopt. Determine which selected flags, including SO_RCVTIMEO and SO_SNDTIMEO, require non-integer values and appropriate structs; the work is done when those flags no longer fail the integer validation and are handled by the native code.

Written by the indexing model from the issue text.