Add support for TLS client certificates to 'erc-tls'

[Amin Bandali]

Tags: patch

After a few recent changes in my IRC/ERC setup, I felt ERC's lack of
support for connecting to IRC networks using TLS client certificates as
an alternative to traditional password-based authentication.  So I spent
some time a few nights ago and wired things up in ERC to allow passing a
client certificate to `open-network-stream'.

Fellow ERC user 'neverwas' was kind enough to take my patch for a test
drive and provide feedback.  I have done some work on addressing their
feedback, but for now I'm attaching my very first draft here which I'd
sent to neverwas, and will attach the next version of my patch (along
with a proper commit message, NEWS entry, etc) shortly after neverwas's
reply with their feedback.

For anyone wanting to take this for a spin, the changes here should be
fully backward-compatible, and should not break anything in existing ERC
configurations.  To specify a client certificate, call `erc-tls' with
the new keywords :client-key and :client-crt with the file name of the
certificate key and certificate file itself respectively:

    (erc-tls :server "chat.freenode.net" :port 6697
             :client-key "~/my-freenode-cert.key"
             :client-crt "~/my-freenode-cert.crt")


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0001-Add-support-for-TLS-client-certificates-to-erc-tls.patch
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