TLS: CA-file override for private CAs

[sidick]

Deferred from the issue #3 TLS implementation plan.

Add a `CAFILE`/`--cafile` escape hatch for brokers behind a private CA that isn't in AmiSSL's bundled trust store — a real scenario for self-hosted setups that want certificate verification without falling back to `-S`/`TLSINSECURE` (which skips verification entirely).

Additive, not structural: a new `mco_CAFile`/`tool_opts` field and an explicit `SSL_CTX_load_verify_locations()` call in `transport_amissl.c`/`transport_openssl.c`, once those exist (issue #3).

[sidick]

Implemented in #18: `CAFILE`/`-c` threaded through both TLS transports,
mqtt.library's `mco_CAFile`, and both CLI flavors, additive to the existing
trust store (not a replacement). Verified end-to-end on real m68k/AmiSSL
under Copperline against a broker signed by a private CA.

Side finding while verifying on-target: the Amiga's system clock matters a
lot for cert verification - a wrong/unseeded clock makes a good cert look
not-yet-valid. Documented in CLI-Reference.md's new "A note on TLS and the
system clock".