Allow the 'file' keyword to be optionally used on file rules.

Add the optional 'file' keyword to the language/grammer.  The main reason
for doing this is to support false token injection.  Which is needed
to move towards the parser being broken out into an api that can be
used to parse individual rule types, separate from parsing the whole file.

Since we are adding the token to the grammar expose it to userspace with
the 'file' keyword.  While not needed it helps bring consistency, as all
the other rule types start with a keyword (capability, network, rlimit, ...).

Also allow the bare keyword to be used to represent allowing all file
operations, just as with network and capability.  Domain transitions are
defaulted to ix.  Thus

  file,

is equivalent to

  /** rwlkmix,

Signed-off-by: John Johansen <john.johansen@canonical.com>
Acked-by: Kees Cook <kees@ubuntu.com>

parser/parser_misc.c

@@ -77,6 +77,7 @@ static struct keyword_table keyword_table[] = {
 	{"alias",		TOK_ALIAS},
 	{"rewrite",		TOK_ALIAS},
 	{"ptrace",		TOK_PTRACE},
+	{"file",		TOK_FILE},
 	/* terminate */
 	{NULL, 0}
 };