include: Move sarg.c stuff to nsc.h to avoid incomplete enum

include: Move sarg.c stuff to nsc.h to avoid incomplete enum

Portability issue: prototypes.h uses enum ns_seltype, defined in
nsc.h.  GCC is fine with that; it supports incomplete enum types as an
extension.  Broken in commit 157ae8ec2 "Clean up superfluous include
of nsc.h in prototypes.h", v4.3.27.  Moving sarg.c stuff to nsc.h
fixes it.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>