src: tunnel src/dst must be a symbolic expression

src: tunnel src/dst must be a symbolic expression

Included bogons crash with segfault and assertion.  After fix:

tunnel_with_garbage_dst:3:12-14: Error: syntax error, unexpected tcp, expecting string or quoted string or string with a trailing asterisk or '$'
  ip saddr tcp dport { }
           ^^^
The parser change restricts the grammar to no longer allow this,
we would crash here because we enter payload evaluation path that
tries to insert a dependency into the rule, but we don't have one
(ctx->rule and ctx->stmt are NULL as expected here).

The eval stage change makes sure we will reject non-value symbols:

tunnel_with_anon_set_assert:1:12-31: Error: must be a value, not set
define s = { 1.2.3.4, 5.6.7.8 }
           ^^^^^^^^^^^^^^^^^^^^

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>