Tune pseudovector allocation assuming Qnil == 0

Tune pseudovector allocation assuming Qnil == 0

* alloc.c (allocate_pseudovector): Use memset for both
Lisp_Objects and regular slots.  Add zerolen arg.
* lisp.h (allocate_pseudovector): Adjust prototype.
(ALLOCATE_PSEUDOVECTOR): Adjust user.
(ALLOCATE_ZEROED_PSEUDOVECTOR): New macro.
(allocate_hash_table, allocate_window, allocate_frame)
(allocate_process, allocate_terminal): Remove prototypes.
* fns.c (allocate_hash_table): Now static here.
* frame.c (allocate_frame):
* process.c (allocate_process):
* terminal.c (allocate_terminal):
* window.c (allocate_window): Now static here.
Use ALLOCATE_ZEROED_PSEUDOVECTOR.  Add comment.