M4 was designed to have a small number of header files defining

M4 was designed to have a small number of header files defining
the interfaces to several source files each.  Keeping this in
mind allows us to tidy things up considerably.  Having done this,
it becomes clear that some of the installed headers reference
other headers which are not installed (but that's another patch):

* m4/m4private.h (stdio--.h, stdlib--.h, unistd--.h): Moved from
here...
* m4/system_.h (gnu/stdio--.h, gnu/stdlib--.h, gnu/unistd--.h):
...to here, so client modules can benefit too.
(assert.h, errno.h, limits.h, sys/stat.h): Include one here at the
lowest level to save doing it multiple times elsewhere.
* m4/debug.c (stdio.h): Removed.  The portable <gnu/stdio--.h>
is already included via m4private.h.
* modules/time.c (stdio.h): Ditto.
* modules/gnu.c (stdlib.h): Ditto resp <gnu/stdlib--.h>.
* modules/stdlib.c (stdlib.h, unistd.h): Ditto resp <gnu/stdlib--.h>
and <gnu/unistd--.h>,
* m4/m4private.h (m4module.h): Use angle brackets for an installed
file.
* m4/hash.h (system.h): Ditto.
* modules/gnu.c (progname.h): Removed.
* src/freeze.c (m4/system.h, m4private.h): Already included vi m4.h.
* m4/m4private.h (assert.h, errno.h): Already included via
m4/m4module.h.
* m4/input.c (ctype.h): Ditto.
* m4/module.c (ltdl.h): Ditto.
* m4/output.c (errno.h, limits.h, stdio.h, sys/types.h, sys/stat.h)
(unistd.h): Ditto.
* m4/path.c (config.h, stdlib.h): Ditto.
* m4/utility.c (config.h): Ditto.
* modules/evalparse.c (assert.h, ctype.h): Ditto.
* modules/gnu.c (assert.h, ctype.h, errno.h): Ditto.
* modules/import.c (assert.h): Ditto.
* modules/m4.c (assert.h, errno.h): Ditto.
* modules/modtest.c (assert.h): Ditto.
* modules/stdlib.c (sys/types.h): DittAo.
* src/m4.h (ctype.h, errno.h, string.h, sys/types.h, ltdl.h):
Ditto.
* src/stackovf.c (assert.h): Ditto.
* modules/gnu.c (m4/m4module.h, m4private.h): Build using the faster
private interfaces when NDEBUG is defined.
* modules/import.c (m4/m4module.h, m4private.h): Ditto.
* modules/load.c (m4/m4module.h, m4private.h): Ditto.
* modules/m4.c (m4/m4module.h, m4private.h): Ditto.
* modules/modtest.c (m4/m4module.h, m4private.h): Ditto.
* modules/mpeval.c (m4/m4module.h, m4private.h): Ditto.
* modules/perl.c (m4/m4module.h, m4private.h): Ditto.
* modules/shadow.c (m4/m4module.h, m4private.h): Ditto.
* modules/time.c (m4/m4module.h, m4private.h): Ditto.
* modules/traditional.c (m4/m4module.h, m4private.h): Ditto.
* src/m4.h (locale.h): Moved from here...
* src/main.h: ...to here.
(m4private.h): Already included via m4.h.
* src/stackovf.c (m4private.h): Ditto.