Simplify net-server.c UDP listener setup

In setup_udp_port_1, remove the haddrbuf parameter and use paddr like
we already do for one of the log messages.  In setup_udp_port, remove
the long switch statement and just look for AF_INET/AF_INET6
addresses.  Split up udp_flags into two booleans for clarity.  Update
the comment in loop_setup_network since we did the "To do" item a long
time ago.
Don't depend on sa_len sockaddr field

In socket-utils.h, replace the socklen macro with an inline function
sa_socklen which always uses the address family, even on platforms
with the sa_len sockaddr field.  This removes the need to set sa_len
in socket addresses we construct.
Avoid using length fields in socket addresses

Remove code to set or reference the length fields of socket addresses
(sa_len/sin_len/sin6_len), since they aren't portable and setting them
is not required.  Remove autoconf tests for those fields which are no
longer used or which were never used.

There is one exception: in localaddr.c, we still neeed to reference
sa_len for the definition of ifreq_size on platforms which have
sa_len.  Leave that behind, along with the autoconf test which defines
SA_LEN.