Deprecate use of flask.h and av_permissions.h.

Also remove all internal uses by libselinux.
This requires deleting the old class/perm string lookup tables
and compatibility code for kernels that predate the /sys/fs/selinux/class
tree, i.e. Linux < 2.6.23.

This also fixes a longstanding bug in the stringrep code; it was allocating
NVECTORS (number of vectors in the legacy av_perm_to_string table, i.e.
the total number of legacy permissions) entries in the per-class perms array
rather than MAXVECTORS (the maximum number of permissions in any
access vector).  Ho hum.  I already fixed this in Android but forgot it
here.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>