RFC: deprecate sys/sysmacros.h inclusion from sys/types.h

[Zack Weinberg]

Andreas may still be running whole-distro rebuild tests, but enough
results came back that I feel fairly confident saying that option B
(remove the #include <sys/types.h> from stdlib.h) is a non-starter.
There are too many (sloppily coded, yes) programs that include stdlib.h
and expect it to expose all the POSIX foo_t types.  That leaves us with
option A (remove the #include <sys/sysmacros.h> from sys/types.h) and
even that is going to break stuff.  So we need a deprecation period.

The attached, lightly tested, patch sets up that deprecation period.
>From an application's perspective, it works like this:

* If you include <sys/sysmacros.h> directly, you get major(), minor(),
and makedev() macros and everything works as it did before, regardless
of whether you also include <sys/types.h> and in what order.

* If you define _SYS_TYPES_NO_SYSMACROS before including any headers,
then <sys/types.h> does not include <sys/sysmacros.h> at all.

* If you include <sys/types.h>, you don't include <sys/sysmacros.h>, and
you don't define _SYS_TYPES_NO_SYSMACROS, then you still get major(),
minor(), and makedev() macros, but they trigger a deprecation warning if
you *use* them.  This warning contains a lengthy explanation of what is
about to change and suggests either including <sys/sysmacros.h> or
defining _SYS_TYPES_NO_SYSMACROS, depending on whether you actually
wanted the dev_t manipulators.

The implementation of all that is regrettably messy, but it works.  I
also took the opportunity to genericize some of the code involved;
sys/sysmacros.h and makedev.c both now live in misc/ instead of
sysdeps/, and there's a new 'bits' header that encapsulates the only
thing that varies between Linux and non-, which is the actual encoding
of a dev_t.  (I'd appreciate extra-careful review of the arithmetic in
the new bits headers; I'm not sure it was ever 100% correct, I may have
messed it up, and there don't seem to be any tests.)

I deliberately didn't say "in the next release" in the deprecation
message, because we might want to give this more than one cycle to bake.

zw


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