Having a single plug_closing() function covering various kinds of
closure is reasonably convenient from the point of view of Plug
implementations, but it's annoying for callers, who all have to fill
in pointless NULL and 0 parameters in the cases where they're not
used.
Added some inline helper functions in network.h alongside the main
plug_closing() dispatch wrappers, so that each kind of connection
closure can present a separate API for the Socket side of the
interface, without complicating the vtable for the Plug side.
Also, added OS-specific extra helpers in the Unix and Windows
directories, which centralise the job of taking an OS error code (of
whatever kind) and translating it into its error message.
In passing, this removes the horrible ad-hoc made-up error codes in
proxy.h, which is OK, because nothing checked for them anyway, and
also I'm about to do an API change to plug_closing proper that removes
the need for them.
There are quite a few of them already, and I'm about to make another
one, so let's start with a bit of tidying up.
The CMake build organisation is unchanged: I haven't put the proxy
object files into a separate library, just moved the locations of the
source files. (Organising proxying as a library would be tricky
anyway, because of the various overrides for tools that want to avoid
cryptography.)
