Get rid of security_context_t and fix const declarations.

The const security_context_t declarations were incorrect;
const char * was intended, not char * const.   Easiest fix is to
replace them all with const char *.  And while we are at it, just
get rid of all usage of security_context_t itself as it adds no value.
typedef left to permit building legacy users until such a time as all are
updated.

Change-Id: I2f9df7bb9f575f76024c3e5f5b660345da2931a7
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>