bundle Danga::Socket and Sys::Syscall

bundle Danga::Socket and Sys::Syscall

These modules are unmaintained upstream at the moment, but I'll
be able to help with the intended maintainer once/if CPAN
ownership is transferred.  OTOH, we've been waiting for that
transfer for several years, now...

Changes I intend to make:

* EPOLLEXCLUSIVE for Linux
* remove unused fields wasting memory
* kqueue bugfixes e.g. https://rt.cpan.org/Ticket/Display.html?id=116615
* accept4 support

And some lower priority experiments:

* switch to EV_ONESHOT / EPOLLONESHOT (incompatible changes)
* nginx-style buffering to tmpfile instead of string array
* sendfile off tmpfile buffers
* io_uring maybe?