6.0_BETA: unistd.h breaks pcc

[Aleksey Cheusov <cheusov%tut.by@localhost>]

Inclusion of unistd.h breaks compilation with pcc.

   /usr/include//unistd.h, line 277: warning: unsupported attribute 
'__returns_twice__'

unistd.h:
   276 #ifndef __LIBC12_SOURCE__
   277 pid_t<-> vfork(void) __RENAME(__vfork14) __returns_twice;
   278 #endif

__returns_twice is also used in setjmp.h and ucontext.h.

It would be nice to fix this in netbsd-6.

-- 
Best regards, Aleksey Cheusov.