Fix #2738: Shorten posixlib uio codepaths & add UioTest

[LeeTibbert]

This PR shortens the code paths for posixlib uio.scala scatter/gather methods readv & writev.
It implements UioTest.scala to verify that the shorter paths work as specified.

The creation of UioTest.scala provided the occasion to reorganize & consolidate SocketTestHelpers.scala,
UdpSocketTest.scala, and Udp6SocketTest.scala. This allows one implementation to be used
in a number of places. An upcoming socket.scala PR will be another user of this now common/shared
code.

[LeeTibbert]

Re-based & passing, modulo usual qemu errors.

I am told that in Japanese zen sand gardens it is
traditional to leave a small imperfection so that one
can appreciate the perfection of the remainder of
the garden. When I get (beyond) frustrated
by the apparently spurious qemu failures, I
try to get in touch with that concept.