Fix pivot_root to support named transitions correctly

Rename the pivotroot rule to pivot_root to match the command and the fn
and fix it to support named transition correctly leveraging the parsing
action used for exec transitions.

Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-By: Steve Beattie <sbeattie@ubuntu.com>

parser/parser_misc.c

@@ -83,7 +83,7 @@ static struct keyword_table keyword_table[] = {
 	{"remount",		TOK_REMOUNT},
 	{"umount",		TOK_UMOUNT},
 	{"unmount",		TOK_UMOUNT},
-	{"pivotroot",		TOK_PIVOTROOT},
+	{"pivot_root",		TOK_PIVOTROOT},
 	/* terminate */
 	{NULL, 0}
 };