sys_accept4() was added in kernel 2.6.28, but ARM was not updated
to include it.  The number and types of parameters is such that
no ARM-specific processing is needed, so wiring up sys_accept4()
just requires defining __NR_accept4 and adding a direct call in
the syscall entry table.

Tested with an EABI 2.6.35 kernel and Ulrich Drepper's original
accept4() test program, modified to define __NR_accept4 for ARM.

Using the updated unistd.h also eliminates a warning then building
glibc (2.10.2 and newer) about accept4() being unimplemented.
Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>