[sr-dev] [kamailio/kamailio] tls: add verify_client support (#2166)

[Armen Babikyan]

Note: See below for more info about motivation of this feature.

#### Pre-Submission Checklist
- [ * ] Commit message has the format required by CONTRIBUTING guide
- [ * ] Commits are split per component (core, individual modules, libs, utils, ...)
- [ * ] Each component has a single commit (if not, squash them into one commit)
- [ * ] No commits to README files for modules (changes must be done to docbook files
in `doc/` subfolder, the README file is autogenerated)

#### Type Of Change
- [ ] Small bug fix (non-breaking change which fixes an issue)
- [ * ] New feature (non-breaking change which adds new functionality)
- [ ] Breaking change (fix or feature that would change existing functionality)

#### Checklist:
- [ ] PR should be backported to stable branches
- [ * ] Tested changes locally
- [ ] Related to issue #XXXX (replace XXXX with an open issue number)

#### Description

This feature aims to replace require_certificate and verify_certificate params with a single option, verify_client: 
* Provides flexibility: require_certificate and verify_certificate are both booleans, so there are only 4 max combinations of params, and only 3 of them make sense (require_certificate=1 and verify_certificate=0 does not).  In contrast, verify_client is a list of enumerated values, which can be more gracefully expanded by adding additional behaviors to the enum.
* Motivation for this feature is to enable optional_no_ca behavior, described in the docbook; Without this feature, that behavior cannot be represented by any combination of require_certificate and verify_certificate.  I figured if I need to add another variable to support desired behavior, it may as well be one that can hold more than just boolean values.
* This feature was inspired from a similar one in Nginx; that software has a similar "ssl_verify_client" option that takes the same "on", "off", "optional", and "optional_no_ca" values, which effectively implement the same feature.  Note that there is no shared code between implementations, and that these behaviors are implemented (in both cases) via a very thin layer of glue code on top of the OpenSSL library.
* Note that the only function definition in tls_verify.c, verify_callback(int pre_verify_ok, X509_STORE_CTX *ctx), has been compiled into the kamailio binary, but apparently not used.  Rather than modify the existing function, I added a simple 1-line function (2 if you count the log message too) to enable this feature.

Please let me know if I can answer any questions.  Thanks!
You can view, comment on, or merge this pull request online at:

  https://github.com/kamailio/kamailio/pull/2166

-- Commit Summary --

  * tls: add verify_client support

-- File Changes --

    M src/modules/tls/doc/params.xml (52)
    M src/modules/tls/tls_cfg.c (3)
    M src/modules/tls/tls_cfg.h (1)
    M src/modules/tls/tls_config.c (31)
    M src/modules/tls/tls_config.h (4)
    M src/modules/tls/tls_domain.c (20)
    M src/modules/tls/tls_domain.h (14)
    M src/modules/tls/tls_mod.c (12)
    M src/modules/tls/tls_rpc.c (3)
    M src/modules/tls/tls_verify.c (6)
    M src/modules/tls/tls_verify.h (5)

-- Patch Links --

https://github.com/kamailio/kamailio/pull/2166.patch
https://github.com/kamailio/kamailio/pull/2166.diff

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