lxc-copy: do not use mkostemp and dprintf

lxc-copy: do not use mkostemp and dprintf

Fixes android builds:

DSBINDIR=\"/data/lxc/lxc/sbin\"      -I/build/libcap/libcap/include/ -Wall -Werror -MT lxc_copy.o -MD -MP -MF $depbase.Tpo -c -o lxc_copy.o lxc_copy.c &&\
	mv -f $depbase.Tpo $depbase.Po
lxc_copy.c: In function 'mount_tmpfs':
lxc_copy.c:834:2: error: implicit declaration of function 'mkostemp' [-Werror=implicit-function-declaration]
  fd = mkostemp(premount, O_CLOEXEC);
  ^
lxc_copy.c:841:2: error: implicit declaration of function 'dprintf' [-Werror=implicit-function-declaration]
  ret = dprintf(fd, "#! /bin/sh\n"
  ^

Signed-off-by: Christian Brauner <cbrauner@suse.de>