signal: Reduce copy_siginfo to just a memcpy

The savings for copying just part of struct siginfo appears to be in the
noise on modern machines.  So remove this ``optimization'' and simplify the code.

At the same time mark the second parameter as constant so there is no confusion
as to which direction the copy will go.

This ensures that a fully initialized siginfo that is sent ends up as
a fully initialized siginfo on the signal queue.  This full initialization
ensures even confused code won't copy unitialized data to userspace, and
it prepares for turning copy_siginfo_to_user into a simple copy_to_user.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>