Inline nnselect helper macros

Inline nnselect helper macros

* lisp/gnus/gnus-registry.el (gnus-nnselect-group-p):
* lisp/gnus/gnus-sum.el (nnselect-article-rsv): Silence byte-compiler.
* lisp/gnus/nnselect.el (nnselect-group-server): New function.
(nnselect-artlist-length):
(nnselect-artlist-article):
(nnselect-artitem-group):
(nnselect-artitem-number):
(nnselect-artitem-rsv):
(nnselect-article-group):
(nnselect-article-number):
(nnselect-article-rsv):
(nnselect-article-id):
(nnselect-categorize):
(ids-by-group):
(numbers-by-group): Inline for speed.