doc: clarify SMACK related --help and documentation

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.