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>
diff --git a/src/callbacks.c b/src/callbacks.c
index b245364..c3cf98b 100644
--- a/src/callbacks.c
+++ b/src/callbacks.c
@@ -32,7 +32,7 @@
 }
 
 static int
-default_selinux_validate(security_context_t *ctx)
+default_selinux_validate(char **ctx)
 {
 	return security_check_context(*ctx);
 }
@@ -59,7 +59,7 @@
 	default_selinux_audit;
 
 int
-(*selinux_validate)(security_context_t *ctx) =
+(*selinux_validate)(char **ctx) =
 	default_selinux_validate;
 
 int