bug#19634: counting MANY function args more reliably

[Paul Eggert]

Commit ef5a526f1b51b76b0f753e0936c80743a7f4463d fixed a bug in Emacs where the C 
code passed 10 arguments to a function using the MANY calling convention, but 
the caller mistakenly passed 8 as the argument count, and this mistakenly 
discarded the last two arguments; see <http://bugs.gnu.org/3228#63>.

To help prevent this sort of mistake in the future, I would like to install 
something like the attached patch, which uses a new C macro CALLN to count these 
arguments automatically.  The key lines in the patch are the following additions 
to lisp.h:

#define CALLMANY(f, array) (f) (ARRAYELTS (array), array)

#define CALLN(f, ...) CALLMANY (f, ((Lisp_Object []) {__VA_ARGS__}))

This lets code use 'return CALLN (foo, a, b, c, d);' instead of the current 
error-prone usages which are like '{ Lisp_Object args[4]; args[0] = a; args[1] = 
b; args[2] = c; args[3] = d; return foo (3, args); }'.  (Oops, that last '3' 
should have been a '4'....)

This patch still needs a ChangeLog entry and some more testing on my part, but I 
thought I'd get it out for review now.

calln.diff
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