Bracket IPv6 address literals in HTTP/nikto/testssl host strings

[RamsesRodenburg]

Fixes #55.

IPv6 port discovery already works (nmap is correctly given -6), but the
HTTP-layer checks build the connection target by plain concatenation —
f"http://{host}:{port}" and f"{host}:{port}". For an IPv6 literal that yields
http://2001:db8::1:443, which is unparseable: RFC 3986 requires the literal to
be bracketed (http://[2001:db8::1]:443, [2001:db8::1]:443). So scanning an
IPv6 target logs:

http://2001:db8::1:80  Could not connect: Failed to parse: http://2001:db8::1:80

and IPv6 hosts get nmap + SSH/TLS results but no HTTP / nikto findings.

This adds a small bracket_ipv6() helper and applies it at the five sites where
the host goes into a URL or host:port string. It uses the same ":" in host
test the script already uses to decide on -6, so IPv4 addresses and hostnames
are unaffected.

Changed (URL / host:port contexts):

• http_checks() — the http:// and https:// URLs
• check_trace() — the curl host:port target
• do_nikto() — the -host host:port argument
• do_testssl() — the host:port target

Left unchanged:

• the nmap target (bare literal + -6, which is correct)
• do_nikto()'s -vhost {host} (Host header) — for strict correctness an IPv6
  Host: header is also bracketed, but that's the request header rather than the
  connection, and I didn't want to assume the bundled nikto's expectation; happy
  to add if you'd like
• the per-finding log prefix (display only)

Testing: built a patched image from gofwd/analyze_hosts with this change and
scanned a real dual-stack host by its IPv6 literal — HTTP checks now connect and
report findings (compression / headers / nikto) where they previously failed
with Failed to parse; nmap and SSH/TLS behaviour unchanged.

[RamsesRodenburg]

Since I have no option/permissions to squash/merge - feel free to do so as you please 😉