aarch64_support #25

* src/fcntl.c (openat): New function.

arm64/aarch64 only has openat and no open.
* src/daemon.c: Replace calls.
* src/unistd.h: Remove open function prototype.
* src/unistd.c: Remove open function definition.

pipe -> pipe2
dup2 -> dup3
fork -> clone3
epoll_wait -> epoll_pwait
unlink -> unlinkat
* src/daemon.c (daemonize): Replace calls; pipe with pipe2, dup2 with dup3 and
 fork with clone3.
* src/main.c (isolate_ns): Replace calls; fork with clone3.
(main): Replace calls; fork with clone3, epoll_wait with epoll_pwait, unlink
with unlinkat.
* src/sched.d (clone3): New function.
* src/sys/epoll.c (epoll_wait): Migrate to epoll_pwait.
* src/unistd.c (fork): Remove function.
(dup2): Migrate to dup3.
(pipe): Migrate to pipe2.
(unlink): Migrate to unlinkat.