bug#21782: 25.0.50; New functions nfront/front

[Tino Calancha]

> I think this is already available in core as seq-take, from the seq package.

Thank you, you are right. Indeed, I found seq-take very nice:
its more general because apply to sequences.
There is one small difference: seq-take has no optional arguments.

In addition, seq.el need to be loaded by the user. Yeah, not a big
deal, but IMO such fundamental operation on a list as nfront 
implement, deserve to be available at the starting of the session.
Putting a dedicated function in subr.el we save to load many stuff from
seq.el that maybe we dont need.

I would like nfront/front would behave as nbutlast/butlast concerning
the N argument:
*) N nil giving same output as N = 1.
*) N <= 0 return the full list.
(I choose the name of the function to somehow reflect such symmetry).

I have modified my patch to accomplish such behaviour.

With the new patch

(let ((ltest '(1 2 3 4 5 6)))
  (nbutlast ltest nil))
(1 2 3 4 5)
(let ((ltest '(1 2 3 4 5 6)))
  (nfront ltest nil))
(1)
(require 'seq)
(let ((ltest '(1 2 3 4 5 6)))
  (seq-take ltest nil)); error: second argument is not optional

(let ((ltest '(1 2 3 4 5 6)))
  (nbutlast ltest 0))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
  (nfront ltest 0))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
  (seq-take ltest 0))
nil

(let ((ltest '(1 2 3 4 5 6)))
  (nbutlast ltest -1))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
  (nfront ltest -1))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
  (seq-take ltest -1))
nil

(let ((ltest '(1 2 3 4 5 6)))
  (nbutlast ltest 20))
nil
(let ((ltest '(1 2 3 4 5 6)))
  (nfront ltest 20))
(1 2 3 4 5 6)
(let ((ltest '(1 2 3 4 5 6)))
  (seq-take ltest 20))
(1 2 3 4 5 6)

subr-2.patch
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