Fix various <sys/socket.h> prototypes.

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_socket.h.html

Interestingly, recvmmsg/sendmmsg do actually return int, unlike their
recv/recvmsg/sendv/sendvmsg siblings.

The generated assembler stubs don't actually change.

Change-Id: If71cd02431903774de9bbce77fbedf6b03f90c0f