Remove unused HAVE_SOCKADDR_SA_LEN ifdef'd code

dnsmasq is only built for Android on Linux.  Clean up a bit so we
can more easily read the code.

Also: a few whitespace fixes and assertion of HAVE_IPV6 code paths.

Test: as follows
    - built, flashed, booted
    - USB tethering works as expected
Bug: 38364896
Change-Id: I636ba74b83594f13a7a2e9836e8829e113ca88a0
7 files changed