Nominating polarssl for deprecation

Hi,

The polarssl TLS library has not had an update in over three years. The last
release was done on January 7 2019 [1]. That library has been superceded by
the mbedtls library, which is the current incarnation of it.

In curl we support both, with two separate implementations.

I propose that we move to DEPRECATE PolarSSL support in curl because it seems
unlikely that this library is a good choice for users to get proper TLS
security and support today and at the same time there are plenty of good and
updated alternatives.

I consider it likely that the existing users of curl + polarssl out there are
stuck on old curl versions and when they eventually manage to update curl they
should also be able to update their TLS library.

If nobody objects, I will file a PR soon that makes the polarssl disabled in
code but still present, and mark it for complete removal six months after the
release where it is marked as deprecated. Possibly then marked deprecated in
the July 17 2019 release, removed from the code after January 17 2020.

Opinions?

[1] = https://tls.mbed.org/tech-updates/releases

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