toke.c: Extract part of \N{} processing to new function

toke.c: Extract part of \N{} processing to new function

This is in preparation for making fatal the deprecations that this code
covers.

This code combines the first and final portions of the code that handles
\N{names}, leaving the middle intact.  There are no intentional logic
changes.  The code is moved and outdented as appropriate for not being
within nested "if's", and the comments are reflowed to fill 79 columns.
One declaration had a const added.

This causes the logic that checks for input name validity to be moved
from after everything is computed to doing it beforehand.  Since invalid
names are not currently fatal, there was no problem with checking them
after computing things, but a future commit will make them fatal, so
this saves the work of computing something that is erroneous.