Constify some socket-utils.h functions

Constify some socket-utils.h functions

Accept constant pointers in sa_getport(), sa_is_inet(),
sa_is_wildcard(), and sa_socklen().

Make sa2sin() and sa2sin6() accept and return constant pointers.  For
the most part we write to sockaddr_storage and read from sockaddr.
The biggest exception is udppktinfo.c; adjust its internal APIs to use
sockaddr_storage for output and inferred lengths for input.  All of
the other exceptions use sa_setport().