ls: optimize for when getfilecon would often fail (~33% perf. gain)

On systems or file systems without SELinux support, all getfilecon
and lgetfilecon calls would fail due to lack of support.  We can non-
invasively cache such failure (on most recently accessed device) and
avoid the vast majority of the failing underlying getxattr syscalls.
* src/ls.c (errno_unsupported): New function.
(selinux_challenged_device): New file-scoped global.
(getfilecon_cache, lgetfilecon_cache): New error-caching wrapper
functions.
(gobble_file): Use the caching wrappers, for when many *getfilecon
calls would fail with ENOTSUP or EOPNOTSUPP.
Suggested by Sven Breuner in
http://thread.gmane.org/gmane.comp.gnu.coreutils.general/2187
Improved-by: Pádraig Brady.

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