Remove AT&T UWIN support code

UWIN was David Korn's UNIX emulation layer for Microsoft Windows.
It was never very well known, certainly not like Cygwin or
Microsoft SFU/Interix. It was a very interesting system that
exposed the Windows registry to the file system, making it
UNIX-like, and that natively used ksh and all the AST utilities.

Regrettably, it appears to be dead and buried. Only 32-bit binaries
can still be found in the wild, as well as the source code at:
	https://github.com/att/uwin
The latter does not seem to be usable since (as far as I can tell)
it requires a UWIN environment with a compiler to build, and UWIN
binaries with a compiler are simply nowhere to be found.

The activity level on that repo (which is zero) also shows how much
interest there still is in this project. And of course the
supporting code in this repo is almost certainly broken by now as
we've never been able to test it on a UWIN system.

The AST team clearly cared about it since roughly 8k lines of code
are dedicated to its support, disabled (directly or indirectly) on
non-UWIN systems via the _UWIN macro. This removes all that.

src/cmd/ksh93/features/externs

@@ -1,7 +1,7 @@
 hdr	nc,exec_attr
 mem	exception.name,_exception.name math.h
 lib	setreuid,setregid,nice,fork,spawnveg,fchdir
-lib	pathnative,pathposix,uwin_path,uwin_unpath,fts_notify
+lib	pathnative,pathposix,fts_notify
 lib	memcntl sys/mman.h
 lib	getexecuser,free_execattr exec_attr.h -lsecdb
 

src/cmd/ksh93/include/path.h

@@ -31,7 +31,7 @@
 #include	"defs.h"
 
 #if !defined(SHOPT_SPAWN)
-#   if _UWIN || _use_spawnveg
+#   if _use_spawnveg
 #	define  SHOPT_SPAWN  1
 #   endif
 #endif /* !SHOPT_SPAWN */

src/cmd/ksh93/sh/name.c

@@ -44,20 +44,6 @@ static Dtdisc_t	_Refdisc =
 	offsetof(struct Namref,np),sizeof(struct Namval_t*),sizeof(struct Namref)
 };
 
-#if !_lib_pathnative && _lib_uwin_path
-
-#define _lib_pathnative		1
-
-extern int	uwin_path(const char*, char*, int);
-
-size_t
-pathnative(const char* path, char* buf, size_t siz)
-{
-	return uwin_path(path, buf, siz);
-}
-
-#endif /* _lib_pathnative */
-
 static void	attstore(Namval_t*,void*);
 static void	pushnam(Namval_t*,void*);
 static char	*staknam(Namval_t*, char*);