Sockets rewrite follow-ups and leftoevers

[boryspoplawski]

After #579 there are still some less important TODOs and cleanup ideas:

• Clean unused stuff from struct handle_ops in Pal/include/pal_internal.h (e.g. readbyaddr).
• Clean up socket/listen/bind/connect ocalls. Currently there are two versions of them (one doing a single syscall, other batching a few) - we need to at least change the names to something more verbose (or remove/rewrite some, needs some thinking and discussion).
• Clean up lists of allowed error codes in ocalls. Currently most of them allow e.g. for EBADF, which should never be the case if we pass correct arguments.
• Add void* private into struct shim_sock_handle - this field would be private for each backend (e.g. IP could store some socket options there). We would probably move reuseaddr to there too.