Reverse the sense of the -D option, from disabling setting/use of
security.restorecon_last to enabling it, making disabled the default state.
Rationale:
1) Users often use restorecon to fix labels on files whose labels are
wrong even through nothing has changed in file_contexts, e.g. after
copying/moving files to a different location. They won't expect
restorecon to suddenly stop relabeling by default because the hash of
file_contexts hasn't changed.
2) Only processes running with CAP_SYS_ADMIN can set
security.restorecon_last, so this will fail for non-root users anyway.
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>