Enhance libnutclient (C++) with SSL (NUT STARTTLS) support and test it

[jimklimov]

Addresses at least parts of:

• libnutclient (C++) does not talk SSL #1599
• CI: actively test SSL setups with NIT #1711

Adds various improvements to the NIT script along the way.

AI DISCLAIMER: Prepared with contributions from IntelliJ Junie, mostly to speed up repetitive coding patterns like getters/setters.

[jimklimov]

Some troubles remain with certificate setup:

• Segfault not propagated as a tooling failure:

/net/donutci-openbsd-65-amd64/var/shm/jenkins-donutci-openbsd-65-amd64/nut_nut_PR-3353/tests/NIT/nit.sh:
  line 959:  7503 Segmentation fault      (core dumped)
    certutil -A -d . -f .pwfile -n "${TESTCERT_SERVER_NAME}" -a -i server.crt -t ",,"

• Duplicate serial?

Is this a CA certificate [y/N]?
Enter the path length constraint, enter to skip [<0 for unlimited path]: > Is this a critical extension [y/N]?
certutil: could not decode certificate: SEC_ERROR_REUSED_ISSUER_AND_SERIAL: You are attempting to import a cert with the same issuer/serial as an existing cert, but that is not the same cert.
-rw------- 1 abuild abuild 28672 Mar 20 11:05 /dev/shm/jenkins-nutci/nut_nut_PR-3353/obj/tests/NIT/tmp/etc/cert/upsd/cert9.db
-rw------- 1 abuild abuild 36864 Mar 20 11:05 /dev/shm/jenkins-nutci/nut_nut_PR-3353/obj/tests/NIT/tmp/etc/cert/upsd/key4.db
-rw------- 1 abuild abuild   408 Mar 20 11:05 /dev/shm/jenkins-nutci/nut_nut_PR-3353/obj/tests/NIT/tmp/etc/cert/upsd/pkcs11.txt
Fri Mar 20 11:05:04 UTC 2026 [INFO] SSL: Preparing test client certificate...
certutil: could not decode certificate: SEC_ERROR_REUSED_ISSUER_AND_SERIAL: You are attempting to import a cert with the same issuer/serial as an existing cert, but that is not the same cert.

...
##Failure Location unknown## : Error
Test name: nut::NutActiveClientTest::test_query_ver
uncaught exception of type std::exception (or derived).
- STARTTLS failed: ERR FEATURE-NOT-CONFIGURED