'RELENG_6_0_0_RELEASE'.

This commit was manufactured to restore the state of the 6.0-RELEASE image.

to mistakes from day 1, it has always had semantics inconsistent with
SVR4 and its successors.  In particular, given argument M:

- On Solaris and FreeBSD/{alpha,sparc64}, it clobbers the old flags
  and *sets* the new flag word to M.  (NetBSD, too?)
- On FreeBSD/{amd64,i386}, it *clears* the flags that are specified in M
  and leaves the remaining flags unchanged (modulo a small bug on amd64.)
- On FreeBSD/ia64, it is not implemented.

There is no way to fix fpsetsticky() to DTRT for both old FreeBSD apps
and apps ported from other operating systems, so the best approach
seems to be to kill the function and fix any apps that break.  I
couldn't find any ports that use it, and any such ports would already
be broken on FreeBSD/ia64 and Linux anyway.

By the way, the routine has always been undocumented in FreeBSD,
except for an MLINK to a manpage that doesn't describe it.  This
manpage has stated since 5.3-RELEASE that the functions it describes
are deprecated, so that must mean that functions that it is *supposed*
to describe but doesn't are even *more* deprecated.  ;-)

Note that fpresetsticky() has been retained on FreeBSD/i386.  As far
as I can tell, no other operating systems or ports of FreeBSD
implement it, so there's nothing for it to be inconsistent with.

PR:		75862
Suggested by:	bde

bit in a long double.  For architectures that don't have such a bit,
LDBL_NBIT is 0.  This makes it possible to say `mantissa & ~LDBL_NBIT'
in places that previously used an #ifdef to select the right expression.
The optimizer should dispense with the extra arithmetic when LDBL_NBIT
is 0.

scalbn() implementation from libm.  (The two functions are defined to
be identical, but ldexp() lives in libc for backwards compatibility.)
The old ldexp() implementation...
  - was more complicated than this one
  - set errno instead of raising FP exceptions
  - got some corner cases wrong
    (e.g. ldexp(1.0, 2000) in round-to-zero mode)

The new implementation lives in libc/gen instead of
libc/$MACHINE_ARCH/gen, since we don't need N copies of a
machine-independent file.  The amd64 and i386 platforms
retain their fast and correct MD implementations and
override this one.

The purpose of having a separate file involved an abandoned scheme that
would have kept contrib/gdtoa out of the include path for the rest of libc.

operation (by subtracting the absolute result from 0), don't test
for overflow.

This avoids an arithmetic exception when dividing LONG_MIN by 1:
This is the only case that causes overflow, and the resulting value
is correct under 2's compliment arithmetic.

PR:		72024
Approved by:	dwmalone@
Obtained from:	NetBSD
MFC after:	4 days

any fake value.

PR: 70518