restorer: Block signals early

Otherwise threads can handle them before calling sigreturn with wrong (not
belonging to crtools, but to the target task) handers:

__export_restore_task				| __export_restore_thread

restore_finish_stage(CR_STATE_RESTORE);		| restore_finish_stage(CR_STATE_RESTORE);
sys_sigaction(SIGCHLD, &args->sigchld_act, ...) |
restore_signals()				|
	ksigfillset(&to_block);			|
	sys_rt_sigqueueinfo(sys_getpid(), ...); |
						| execute signal handler() <------ BUG !!!
						| restore_signals()
						|	ksigfillset(&to_block);

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>