fix x86-64 cpuid test to call geteuid
fix cpuid.run to be multi-architecture aware
separate syscall buffering from vdso monkeypatching

Fixes #1345.
make accept4_args inherit from accept_args
factor out a prepare_accept function

I think this also fixes a small bug in the accept4 implementation: we're
supposed to determine how much space to allocate for the addr based on
the in-value of addrlen.  The accept implementation was doing that, but
the accept4 implementation was allocating space for a bare |sockaddr|.
Probably didn't matter much in practice, though.
record and replay the accept{,4} syscalls
add support for the recvfrom syscall
Reformat