Provide fine-grained keepalive options on Linux, which
supports per-socket defaults.

Fix a bug with trying to set keepalive options for UDP
sockets - don't try to do it.

socket.tcp_server() changes:

 * Set nice fibers names for listener and clients
 * Use only one address for bind
 * Return single socket instead of wrapper
 * Add opts = {} to arguments
 * Automatically remove UNIX domain socket on server:close()

Other changes:

 * Fix buggy port number parsng for ipv6
 * Add peer address to accept() results like recvfrom()

Fixes compatibility with third-party modules.

This patch also adds argument checking to all socket methods.

(fix httpd log messages)

(so user can use 'SO_REUSEADDR' option in tcp_server)