label: when clearing selinux context, don't mangle errno
tmpfiles: various modernizations
tmpfiles: add new "C" line for copying files or directories
tmpfiles: get rid of "m" lines, make them redundant by "z"

"m" so far has been a non-globbing version of "z". Since this makes it
quite redundant, let's get rid of it. Remove "m" from the man pages,
beef up "z" docs instead, and make "m" nothing more than a compatibility
alias for "z".