Add accept4() syscall

Add accept4() using SYSCALLS.TXT and gensyscall

Change-Id: I6f19f29144186d15d46423e10f2cc4b4223719c6

Conflicts:
	libc/SYSCALLS.TXT
	libc/include/sys/socket.h
Small accept4 fixes.

Put the accept4 test in the sorted order, and put the accept4 define in
sorted order.

Also add the missing SYS_RECVMMSG and SYS_SENDMMSG defines.

Change-Id: Iba55354975e0d5027dbee53f6de752c2df719493

Conflicts:
	libc/include/sys/socket.h
	tests/Android.mk
Clean up CMSG_*HDR in <sys/socket.h>

Removed __CMSG_NXTHDR, __CMSG_FIRSTHDR and cmsg_nxthdr.

Bug: 13418328
Change-Id: I6b1ed58b41b3f37d41e070c4f222b5bfa512bc48

Conflicts:
	libc/include/sys/socket.h
Use glibc structure for __cmsg_nxthdr.

Bug: 13418328
Change-Id: I7e656b373e53e80d708325321e48dcedacf5086d

Conflicts:
	libc/Android.mk
	libc/include/sys/socket.h