MFC r281437 (by mjg@):

Replace struct filedesc argument in getsock_cap with struct thread

This is is a step towards removal of spurious arguments.

MFC r272823:

Move the SCTP syscalls to netinet with the rest of the SCTP code. The
syscalls themselves are tightly coupled with the network stack and
therefore should not be in the generic socket code.

The following four syscalls have been marked as NOSTD so they can be
dynamically registered in sctp_syscalls_init() function:
sys_sctp_peeloff
sys_sctp_generic_sendmsg
sys_sctp_generic_sendmsg_iov
sys_sctp_generic_recvmsg

The syscalls are also set up to be dynamically registered when COMPAT32
option is configured.

As a side effect of moving the SCTP syscalls, getsock_cap needs to be
made available outside of the uipc_syscalls.c source file. A proper
prototype has been added to the sys/socketvar.h header file.

API tests from the SCTP reference implementation have been run to ensure
compatibility. (http://code.google.com/p/sctp-refimpl/source/checkout)

MFC r286836:

so_vnet is constant after creation and no locking is necessary,
document this fact.

MFC r286836:

so_vnet is constant after creation and no locking is necessary,
document this fact.