shrunk cfgobj down to 48bytes

shrunk cfgobj down to 48bytes

Make all non-scalar properties of `cfg_obj_t` allocated values, which
ensures the union size is the width of one pointer. Also reorder the
fields inside `cfg_obj_t` to avoid alignment padding that would increase
the size. As a result, a `cfg_obj_t` instance is now 48 bytes on a
64-bit platform.

Add a static assertion to avoid increasing the size of the struct by
mistake.

The function `parse_sockaddrsub` was taking advantage of the fact that
both sockaddr and sockaddrtls were in the same position, and used to
initialize the sockaddr field independently if this was a -tls one or
not. This doesn't work anymore now that all fields are allocated,
so it has been slightly rewritten to take both cases into account
separately.