Improve year-2038 documentation

Improve year-2038 documentation

* NEWS, doc/autoconf.texi (System Services):
Improve documentation for behavior of largefile and year-2038 support.
Say that in the current implementation, year-2038 support
requires largefile support.  Say that year-2038 support
matters only for GNU/Linux glibc 2.34+ on 32-bit x86 and ARM.
Prefer brevity when this does not hurt understandability;
for example, prefer active to passive voice.
Prefer “wider” to “larger” when talking about the number of
bits in an integer, as this terminology is more standard.
Tone down the wording in warnings about enabling year-2038 support,
use similar wording in warnings about enabling largefile support,
and warn also about disabling largefile and year-2038 support.
No need for @emph.  Also mention rlim_t.
Be a bit more careful about saying “2 GiB” rather than “2 GB”.
Mention that a future version of Autoconf might change
AC_SYS_LARGEFILE to default to --enable-year2038, since
something has gotta happen before 2038.
Coalesce descriptions of --enable-largefile and --enable-year2038
to simplify documentation.  Mention that the only system where
AC_SYS_LARGEFILE changes CC is IRIX and that these systems
are obsolete.  Say that ‘stat’ can fail due to time_t
overflow.  Say that you can’t portably print time_t with %ld.
Say that binary compatibilty problems also can occur when one
library is linking to amother; it’s not just apps vs libraries.
Mention the possibility of modifying libraries to support both
32- and 64-bit interfaces.  Warn more consistently about
ABI compatibility issues, but put the bulk of this text
in one location that the other locations refer to.