build: simplify: get rid of some indirection variables

build: simplify: get rid of some indirection variables

The code deciding which coreutils programs to build (depending on
defaults, system capabilities, and user requests) is overly complex
and rather confusing.  Let's begin simplifying it by removing some
non-strictly-necessary indirection variables.

* configure.ac: Adjust and improve few comments.
(OPTIONAL_BIN_PROGS, OPTIONAL_PKGLIB_PROGS): Rename ...
(bin_PROGRAMS, pkglibexec_PROGRAMS): ... like these, respectively.
Ensure they aren't initialized in all Makefiles (which would lead
to spurious errors), by calling AM_SUBST_NOTMAKE on them.
* src/local.mk: Adjust and improve few comments.
(bin_PROGRAMS, pkglibexec_PROGRAMS): Simply define
to the corresponding '@substitution@'.