A quicker check for quit

A quicker check for quit

On some microbenchmarks this lets Emacs run 60% faster on my
platform (AMD Phenom II X4 910e, Fedora 25 x86-64).
* src/atimer.c: Include keyboard.h, for pending_signals.
* src/editfns.c (Fcompare_buffer_substrings):
* src/fns.c (Fnthcdr, Fmemq, Fmemql, Fassq, Frassq, Fplist_put)
(Fnconc, Fplist_member):
Set and clear immediate_quit before and after loop instead of
executing QUIT each time through the loop.  This is OK for loops
that affect only locals.
* src/eval.c (process_quit_flag): Now static.
(maybe_quit): New function, containing QUIT’s old body.
* src/fns.c (rarely_quit): New function.
(Fmember, Fassoc, Frassoc, Fdelete, Fnreverse, Freverse)
(Flax_plist_get, Flax_plist_put, internal_equal, Fnconc):
Use it instead of QUIT, for
speed in tight loops that might modify non-locals.
* src/keyboard.h (pending_signals, process_pending_signals):
These belong to keyboard.c, so move them here ...
* src/lisp.h: ... from here.
(QUIT): Redefine in terms of the new maybe_quit function, which
contains this macro’s old definiens.  This works well with branch
prediction on processors with return stack buffers, e.g., x86
other than the original Pentium.