Mark functions as static and arguments/arrays as const where possible

Mark most of functions that are used within only one file as static.
After 95ee490bbdb97ab2b4f1dfa63a0a26e0dd1c2f17 it's easy to prove they
are not in public API.
Several arrays and structs are also marked static.
This prevents them from being exported from liblxc.so

List of removed previously exported symbols:
bdevs
btrfs_ops
check_autodev
create_partial
dir_ops
dump_stacktrace
get_mapped_rootid
get_next_index
lock_mutex
loop_ops
lvm_ops
lxc_abort
lxcapi_clone
lxc_attach_drop_privs
lxc_attach_get_init_uidgi
lxc_attach_getpwshell
lxc_attach_remount_sys_pr
lxc_attach_set_environmen
lxc_attach_to_ns
lxc_clear_saved_nics
lxc_config_readline
lxc_devs
lxc_free_idmap
lxc_global_config_value
lxc_poll
lxc_proc_get_context_info
lxc_set_state
lxc_spawn
mk_devtmpfs
mount_check_fs
ongoing_create
overlayfs_destroy
overlayfs_ops
prepend_lxc_header
remove_partial
save_phys_nics
setup_pivot_root
signames
static_mutex
thread_mutex
unlock_mutex
unpriv_assign_nic
zfs_ops

Signed-off-by: Andrey Mazo <mazo@telum.ru>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>