[Feature] Expose milter headers in legacy RSPAMC protocol

[vstakhov]

Summary

• Expose milter add/remove headers in the legacy RSPAMC and SPAMC text protocol output
• Add extended symbol info (descriptions and options) via X-Symbol: lines
• Backward compatible: existing Symbol: lines unchanged

Problem

Legacy RSPAMC clients (notably Exim via spam = nobody:true with variant=rspamd) only see scores, actions, and symbol names. Milter headers (X-CTCH-, X-CM-, X-Spamd-Result, etc.) and symbol options/descriptions are invisible — they only appear in the JSON response used by HTTP clients.

This blocks Exim users from accessing custom headers added by modules (e.g., Cyren/CTASD, Cloudmark) for false-positive/false-negative reporting.

New protocol lines

Milter header operations

X-Milter-Add: Header-Name: value             # append header
X-Milter-Add: Header-Name[N]: value          # insert at position N
X-Milter-Del: Header-Name                    # remove all instances
X-Milter-Del: Header-Name[N]                 # remove Nth instance

Extended symbol info

X-Symbol: BAYES_SPAM(5.00); Bayes spam probability
X-Symbol: RBL_SPAMHAUS(3.00); Spamhaus RBL [127.0.0.2, sbl.spamhaus.org]
X-Symbol: CTCH_SPAM(5.00); Cyren CTCH [refid:str123]

Format: X-Symbol: NAME(SCORE); DESCRIPTION [OPT1, OPT2, ...]

Exim usage example

warn
  spam = nobody:true
  set acl_m_report = ${sg{$spam_report}{\\v\\s+}{\n}}

  # Add milter headers: filter X-Milter-Add lines, strip prefix + optional [N]
  set acl_m_milter_add = ${sg{\
    ${sg{$acl_m_report}{(?m)^(?!X-Milter-Add: ).*(\n|$)}{}}}\
    {(?m)^X-Milter-Add: ([^\[:\n]+)(?:\[\d+\])?: }{$1: }}
  add_header = $acl_m_milter_add

  # Remove milter headers: filter X-Milter-Del lines, strip prefix + optional [N]
  set acl_m_milter_del = ${sg{\
    ${sg{$acl_m_report}{(?m)^(?!X-Milter-Del: ).*(\n|$)}{}}}\
    {(?m)^X-Milter-Del: ([^\[\n]+).*}{$1}}
  remove_header = $acl_m_milter_del

Test plan

• Build and run unit tests
• Test with rspamc CLI (SYMBOLS method) and verify new lines appear
• Test with Exim variant=rspamd and verify $spam_report contains the new lines
• Verify existing Symbol: lines unchanged (backward compatibility)
• Test with milter_headers module producing both add and remove operations