alpha: use alpha_ni_syscall only for syscall zero

Once upon a time it used to have a C part that printed a warning
about unimplemented OSF syscalls.  That's what it's been doing
all over the OSF syscall range, while the native Linux syscall
range uses sys_ni_syscall().

With those warnings about unimplemented OSF syscalls gone (circa 2.4),
alpha_ni_syscall() has shrunk to that little bit of asm and the
only reason it hasn't been replaced with sys_ni_syscall() everywhere
is that extra twist needed in case of syscall #0.

Let's keep it only for syscall #0 and replace the rest with sys_ni_syscall.
And use sys_ni_syscall for "number out range" in ptraced-call case, as
we'd been doing for normal codepath since 2.1.86...

Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>