Consolidate the 128-bit long double defintions to <sys/ieee754.h>
Each arch that uses it now defines __HAVE_LONG_DOUBLE to 128.
<machine/ieee.h> is now just include the machine's math.h followed
by <sys/ieee754.h>

branches: 1.7.112; 1.7.122; 1.7.128;
merge ktrace-lwp.

It turns out the hppa ABI we're using does not support an extended-
precision type, so drop the library support for now but leave in place
(#ifdef _LP64) the header definitions.

#define __HAVE_LONG_DOUBLE on platforms which implement a distinct
`long double' type.

G/c the local declaration of __nanf; handling of this (conditional on
__HAVE_NANF) was moved to <math.h> some time ago.

Change headers to comply with the new toolchain changes.

branches: 1.2.2;
Add a new feature-test macro, _NETBSD_SOURCE. If this is defined
by the application, all NetBSD interfaces are made visible, even
if some other feature-test macro (like _POSIX_C_SOURCE) is defined.
<sys/featuretest.h> defined _NETBSD_SOURCE if none of _ANSI_SOURCE,
_POSIX_C_SOURCE and _XOPEN_SOURCE is defined, so as to preserve
existing behaviour.

This has two major advantages:
+ Programs that require non-POSIX facilities but define _POSIX_C_SOURCE
can trivially be overruled by putting -D_NETBSD_SOURCE in their CFLAGS.
+ It makes most of the #ifs simpler, in that they're all now ORs of the
various macros, rather than having checks for (!defined(_ANSI_SOURCE) ||
!defined(_POSIX_C_SOURCE) || !defined(_XOPEN_SOURCE)) all over the place.

I've tried not to change the semantics of the headers in any case where
_NETBSD_SOURCE wasn't defined, but there were some places where the
current semantics were clearly mad, and retaining them was harder than
correcting them. In particular, I've mostly normalised things so that
_ANSI_SOURCE gets you the smallest set of stuff, then _POSIX_C_SOURCE,
_XOPEN_SOURCE and _NETBSD_SOURCE in that order.

Tested by building for vax, encouraged by thorpej, and uncontested in
tech-userlevel for a week.

branches: 1.1.2; 1.1.4;
Added files to support generic HP PA-RISC based machines. hp700-specific
files to follow.