Add accept4(), handle fallback from accept() to accept4()

Add accept4(), handle fallback from accept() to accept4()

Add support for the accept4() system call.

Some architectures now have accept4() but not accept(), so introduce a
standard fallback.  However, since accept() is a socketcall, we have
to do some special hacks.

While we are at it, handle conditional socketcall stubs based on
their existence <linux/net.h>, analogous to <asm/unistd.h>.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Gbp-Pq: Name klibc-add-accept4-handle-fallback-from-accept-to-acc.patch