Define global TLSVersion in IdSSL

[Bi0T1N]

this allows to have one set of TLS versions for all SSL/TLS IO handler

[rlebeau]

I'm OK with moving these types into the IdSSL.pas unit. But rather than removing them from the IdSSLOpenSSL.pas unit and risk breaking any existing user code, I would leave them in the IdSSLOpenSSL.pas unit for now and have them alias the new types in IdSSL.pas.

[Bi0T1N]

I've reverted the changes and called the type TLS versions as this is more appropriate.

[rlebeau]

Renaming TIdSSLVersion/s to TIdTLSVersion/s is one thing, but I don't see the need for an explicit tlsvTLS_flexible value at all. Enabling multiple TLS versions at a time should imply the use of "flexible" internally. Just as for OpenSSL, enabling multiple SSL/TLS versions at a time uses SSLv23/TLSMethod internally. And by removing the SSL versions from the enum, the enum can no longer be used with TIdSSLIOHandlerSocketOpenSSL since SSLv2/v3 still need to be supported for OpenSSL 1.0.2 and earlier. Yes, there are still some systems using those old versions. Only OpenSSL 1.1.x drop SSLv2/v3 completely.

[Bi0T1N]

Firstly I named it tlsvTLS_method but to know the meaning you need to read the documentation for it. So I thought naming it tlsvTLS_flexible (or tlsvTLS_highest) is more intuitive as it negotiates the highest possible version between client and server (therefore flexible).
The define is also not meant for use with OpenSSL 1.0.2 (it's EOL and should therefore be ignored for new code) and almost all libraries removed SSLv3 a long time ago. No need to keep it in the possible versions.
I've created the types so that all new SSL IO handler can make use of it to avoid having different schemes for different IO handler. (OpenSSL, LibreSSL, LibTLS, ...)