tests: improve test for a working setfacl

Prompted by a test framework failure of tests/mkdir/p-acl.sh on armv7l:
The previous test for a working setfacl was not sufficient in some
circumstances.

* init.cfg (require_setfacl_): Call setfacl twice with conflictive
ACL specs, and use ACL specs which can't be mapped into regular file
permission bits.  Document the reasons.
copy: fix SELinux context preservation for existing directories

* src/copy.c (copy_internal): Use the global process context
to set the context of existing directories before they're populated.
This is more consistent with the new directory case, and fixes
a bug for existing directories where we erroneously set the
context to the last copied descendent, rather than to that of
the source directory itself.
* tests/cp/cp-a-selinux.sh: Add a test for this case.
* NEWS: Mention the fix.
* THANKS.in: Add reporter Michal Trunecka.
maint: tests: refactor gcc commands for building shared lib

* init.cfg (gcc_shared_): A new function refactored from tests.
(require_gcc_shared_): Adjust to call gcc_shared_() to build the
test library, and remove that library before the function returns.
* tests/cp/nfs-removal-race.sh: Call the new gcc_shared_().
* tests/df/no-mtab-status.sh: Likewise.
* tests/df/skip-duplicates.sh: Likewise.
* tests/ls/getxattr-speedup.sh: Likewise.
* tests/rm/r-root.sh: Likewise.
copy: fix a segfault in SELinux context copying code

* src/selinux.c (restorecon_private): On ArchLinux the
`fakeroot cp -a file1 file2` command segfaulted due
to getfscreatecon() returning a NULL context.
So map this to the sometimes ignored ENODATA error,
rather than crashing.
* tests/cp/no-ctx.sh: Add a new test case.
* tests/local.mk: Reference the new test.
* NEWS: Mention the fix.
Fixes http://bugs.gnu.org/16335
tests: restrict a recent SELinux test to SELinux systems

* tests/cp/no-ctx.sh: Since the test diagnoses whether the
intercepted lgetfilecon() calls are actually called or not,
restrict the test to systems where that occurs.
The test cases are minimal on non SELinux systems and should
be well covered by other tests.
Reported-by: Bernhard Voelker
ln: fix replacing symbolic links whose targets can't exist

* src/ln.c (errno_nonexisting): A new function to determine if
the errno implies that a file doesn't or can't (currently) exist.
(target_directory_operand): Use the new function to expand the
set of errors we handle.
* tests/ln/sf-1.sh: Add test cases for the newly handled errors.
* THANKS.in: Mention the reporter.
* NEWS: Mention the bug fix.
doc: clarify SMACK related --help and documentation

* src/ls.c: Remove all mention of SELinux since ls
should treat all security context labels equally.
* doc/coreutils.texi (ls invocation): Likewise.
(id invocation): Clarify that -Z outputs the context
inherited by the process, rather than one specific to a user.
Note for SMACK this can be set instead by the SMACK64EXEC label,
in the unusual case where this is set on the id executable.
* src/id.c (usage): Likewise.
* src/mkdir.c (usage): Clarify that -Z is specific to SELinux,
while --context=CTX is also supported for SMACK.
* src/mkfifo.c (usage): Likewise.
* src/mknod.c (usage): Likewise.
build: avoid unneeded spawn-pipe gnulib module

* bootstrap.conf: Reference the used pipe-posix module
rather than the uneeded and larger spawn-pipe module,
which the pipe module is the deprecated equivalent of.
build: remove no longer used spawn-pipe files from POTFILES.in

This should have been part of the previous commit v8.22-14-gf18999b

* po/POTFILES.in: Remove gnulib files no longer used.
Prompted by the continuous integration build failure at:
http://hydra.nixos.org/build/8288209/