Add the safe xtransition key word

Currently apparmor provides the unsafe keyword to indicate an xtransition
is not scrubbing its environment variables.  This can be used to be
explicit about which transition are unsafe instead of relying on people
remembering which of px Px is safe or unsafe.

Add the orthogonal keyword safe to allow specifying a transition is
safe.

Signed-off-by: John Johansen <john.johansen@canonical.com>

parser/parser_misc.c

@@ -63,6 +63,7 @@ static struct keyword_table keyword_table[] = {
 	{"defined",		TOK_DEFINED},
 	{"change_profile",	TOK_CHANGE_PROFILE},
 	{"unsafe",		TOK_UNSAFE},
+	{"safe",		TOK_SAFE},
 	{"link",		TOK_LINK},
 	{"owner",		TOK_OWNER},
 	{"user",		TOK_OWNER},