Move some functions and definitions from uipc_socket2.c to uipc_socket.c:

• Move sonewconn(), which creates new sockets for incoming connections on listen sockets, so that all socket allocate code is together in uipc_socket.c.

• Move 'maxsockets' and associated sysctls to uipc_socket.c with the socket allocation code.

• Move kern.ipc sysctl node to uipc_socket.c, add a SYSCTL_DECL() for it to sysctl.h and remove lots of scattered implementations in various IPC modules.

• Sort sodealloc() after soalloc() in uipc_socket.c for dependency order reasons. Statisticize soalloc() and sodealloc() as they are now required only in uipc_socket.c, and are internal to the socket implementation.

After this change, socket allocation and deallocation is entirely
centralized in one file, and uipc_socket2.c consists entirely of socket
buffer manipulation and default protocol switch functions.

MFC after: 1 month