ls: with -Z, show SMACK security context

ls: with -Z, show SMACK security context

Enable showing of file SMACK security with '-Z' command-line switch
if SMACK is enabled.  Showing SMACK context of a file does not strictly
require SMACK to be enabled but this required to make choice whether to
show SELinux or SMACK security context.

* src/ls.c (getfilecon_cache): Retrieve SMACK context if available.
(gobble_file): Handle SMACK context similarly to SELinux context.
* src/local.mk: Link lsl with libsmack.
* NEWS: Mention the new feature.
* .mailmap: Merge the Author's 2 email addresses.