Simplify and modernize strerror substitute, partly to fix Solaris 8 bug.

Simplify and modernize strerror substitute, partly to fix Solaris 8 bug.

* lib/strerror.c: Include <string.h> always, to test interface,
and to remove the need for the dummy.
Include intprops.h to compute width instead of doing it ourselves
and missing a CHAR_BIT declaration, which broke tar 1.19 on Solaris 8.
(strerror): Define it to return NULL if there's no system strerror.
(rpl_strerror): Use INT_STRLEN_BOUND to compute bound.
Omit !HAVE_STRERROR code.  We don't need to worry about supporting
ancient pre-strerror Unix systems well any more.  Saying "unknown
system error" is enough.
* lib/string.in.h (strerror): Simplify the ifdef to reflect the
simpler strerror.c implementation.
* m4/strerror.m4 (gl_FUNC_STRERROR_SEPARATE, gl_PREREQ_STDERROR):
Simplify the tests to reflect the simpler strerror implementation.
* modules/strerror (Depends-on): Add intprops.