res_pjproject: Fix DTLS client check failing on some platforms

res_pjproject: Fix DTLS client check failing on some platforms

Certain platforms (mainly BSD derivatives) have an additional length
field in `sockaddr_in6` and `sockaddr_in`.
`ast_sockaddr_from_pj_sockaddr()` does not take this field into account
when copying over values from the `pj_sockaddr` into the `ast_sockaddr`.
The resulting `ast_sockaddr` will have an uninitialized value for
`sin6_len`/`sin_len` while the other `ast_sockaddr` (not converted from
a `pj_sockaddr`) to check against in `ast_sockaddr_pj_sockaddr_cmp()`
has the correct length value set.

This has the effect that `ast_sockaddr_cmp()` will always indicate
an address mismatch, because it does a bitwise comparison, and all DTLS
packets are dropped even if addresses and ports match.

`ast_sockaddr_from_pj_sockaddr()` now checks whether the length fields
are available on the current platform and sets the values accordingly.

Resolves: #505