Add EICAR test file send via --eicar flag

[steffenfritz]

Summary

• Adds a new -e / --eicar flag that sends the EICAR standard test string as an email attachment (eicar.com) to the target mail server on port 25
• The SMTP server response (accepted / rejected) is reported neutrally — no claim is made about AV filtering, since post-acceptance processing is asynchronous and not visible to the client
• The EICAR string is assembled at runtime from two substrings to prevent false-positive AV detection in the mxcheck binary and source files

Notes

• Only runs when port 25 is open and reachable (fits naturally into the existing port scan loop)
• Uses the existing --mailfrom / --mailto flags for sender and recipient
• No new dependencies

Test plan

• Run mxcheck -s <domain> -e -n against a known test server and verify the "EICAR AV Test" section appears in output
• Verify accepted by server is reported when the server accepts the DATA command
• Verify rejected by server and the server response are reported when the server rejects during DATA
• Run go vet ./... and go test -v ./... — all green
• Confirm no AV tool flags the mxcheck binary after build

Closes #37

🤖 Generated with Claude Code

[steffenfritz]

Closing this PR. In practice, mxcheck runs on workstations or laptops with residential IP addresses, which are blocked by most mail servers before the DATA phase — regardless of content. The EICAR string never reaches the AV filter. Sending from a real MTA account would require SMTP auth support, which is a separate, larger feature. Closing in favor of a more focused discussion in #37.