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)

Submitted by:	Steve Kiernan <stevek@juniper.net>
Reviewed by:	tuexen, rrs
Obtained from:	Juniper Networks, Inc.
Regenerate after r272823:
Move the SCTP syscalls to netinet with the rest of the SCTP code.

Submitted by:	Steve Kiernan <stevek@juniper.net>
Reviewed by:	tuexen, rrs
Obtained from:	Juniper Networks, Inc.