Create new syscalls for mmap(), lseek(), pread(), pwrite(), truncate() and

ftruncate(), but without the pad arg.

There are several reasons for this.  Consider 'mmap()'.  On AMD64, the
function call (and syscall) ABI allow for 6 register arguments.  Additional
arguments go on the stack.  mmap(2) has 6 arguments.  However, the syscall
definition has an extra 'int pad' argument.  This pushes it to 7 arguments,
which means one must spill into the memory stack.  Since the kernel API
doesn't match userland API, we have a hack in libc - libc/sys/mmap.c.
This implements the userland API by calling __syscall() with an extra
argument and the pad argument, for a total of 8 args.  This is all
unnecessary and inconvenient for several things, including the kernel's
syscall handler code which now has to handle merging stack arguments with
register arguments.  It is a big deal for certain 3rd party code.

I'm adding libc glue to make the transition totally painless.  I had
intended to mark the old syscalls as COMPAT6, but the potential to shoot
your feet by building a new kernel without COMPAT_FREEBSD6 but with a
slighly older userland was too great.  For now, they have manual
"freebsd6_" prefixes rather than being COMPAT6.  They will go back to
being marked 'COMPAT6' after 7-stable starts.

Approved by: re (kensmith)

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