Consistently use socklen_t for getsockname, getsockopt etc.

Consistently use socklen_t for getsockname, getsockopt etc.

This fixes signedness mismatch warnings on platforms where socklen_t
is unsigned, notably Linux (where it's an unsigned int).

We still use int for the fallback case where the platform does not
define socklen_t, because that was the traditional (pre-POSIX) type:
for details see NOTES in Linux accept(2),
<http://manpages.debian.org/cgi-bin/man.cgi?query=accept&sektion=2>.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=93069
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>