utime: Fix a test failure on macOS 10.13.
Reported by Martin Storsjö <martin@martin.st> in
<https://lists.gnu.org/archive/html/bug-gnulib/2020-12/msg00003.html>.
* m4/utime.m4 (gl_FUNC_UTIME): Test whether utime handles trailing
slashes on files.
* lib/utime.c (utime): Add alternative implementation for Unix
platforms.
* modules/utime (Depends-on): Add stat.
* doc/posix-functions/utime.texi: Mention the macOS 10.13 bug.
* doc/posix-functions/lstat.texi: Mention that macOS 10.13 also has the
trailing-slash bug.
* doc/posix-functions/open.texi: Likewise.
* doc/posix-functions/stat.texi: Likewise.
* doc/posix-functions/symlink.texi: Likewise.