Fix 'forwarding-options dhcp-security' parse warning for Juniper

[saidvandeklundert]

A 'Parse warning' is being triggered by the following configuration:

set vlans <name> forwarding-options dhcp-security

The dhcp-security configuration is valid only under the ‘forwarding-options’ inside the vlans stanza.

This PR addresses the parse warning by extending the lexer/parser.

The configuration statement is documented here

[batfish-bot]

This change is 

[dhalperi]

Because I'm lazy, I'll just point you at the review I left you on the prior PR https://reviewable.io/reviews/batfish/batfish/8827 :)

[saidvandeklundert]

Updated the description of the PR.

Removed lexer/parser rules for the grammar outside the vlans hierarchy since the Juniper syntax is not valid anywhere else.

Also worked on the other comments by reworking the parser rules and the naming of the rules.

[dhalperi]

Reviewed 2 of 4 files at r1, 1 of 2 files at r2, 1 of 1 files at r3, all commit messages.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @saidvandeklundert)

---

projects/batfish/src/main/antlr4/org/batfish/grammar/flatjuniper/FlatJuniperParser.g4 line 183 at r3 (raw file):

;

vlt_forwarding_options

can you move this code block to be located after vlt_filter in the file?

---

projects/batfish/src/main/antlr4/org/batfish/grammar/flatjuniper/FlatJuniperParser.g4 line 192 at r3 (raw file):

;

vlt_fo_null

vltfo -- the prefix is "underscore-less" so that it's clear where the token being recognized begins.

---

projects/batfish/src/main/antlr4/org/batfish/grammar/flatjuniper/FlatJuniperParser.g4 line 206 at r3 (raw file):

    | vlt_description
    | vlt_filter
    | vlt_forwarding_options    

nit: extra whitespace here.

[dhalperi]

projects/batfish/src/main/antlr4/org/batfish/grammar/flatjuniper/FlatJuniperParser.g4 line 206 at r3 (raw file):

Previously, dhalperi (Dan Halperin) wrote…

nit: extra whitespace here.

(four trailing spaces)

[saidvandeklundert]

Code block moved, vltfo_ renamed according to the standard, trailing spaces removed and ran code formatter.

[dhalperi]

Reviewed 1 of 1 files at r4, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @saidvandeklundert)