[rt.cpan.org 87721] Clear $! in Driver::SysRW::flush().

Nothing prevents the user from calling flush() when there is nothing
to flush.  In these cases, $! remained unchanged, which could lead to
detection of false error conditions.  This change clears $! to avoid
the problem.
[rt.cpan.org 87922] Don't crash when ClientPreConnect returns undef.

ClientPreConnect should be allowed to return an undefined value to
abort the client connection.  Instead, it caused PoCo::Server::TCP to
crash.  This change catches that condition.
Make a new POE::Queue::Array method private.

It's currently experimental, and it currently breaks other event queue
nimplementations, so it has to go.  Making it private will temporarily
reduce temptation to use it, and it helps the POD coverage tests pass.
Remove unused code from POE::Resource::Clock.

A number of functions aren't being used.  More are only being used
privately within the module.  Comment out some of the unused ones
(more dead code checking is needed).  Add leading underscores to the
private ones.
Skip POD::Coverage checks for POE::Pipe::OneWay and TwoWay.

The documentation has been replaced by deprecations.  POD coverage is
moot.