[Nut-upsuser] 2.7.4 uses deprecated back level of TLS

[Roger Price]

2.7.4 explicitly calls for use of a deprecated back level of TLS.

On Debian stretch, after ./configure --with-openssl, make produces the following 
warning:

    upsclient.c: In function 'upscli_init':
    upsclient.c:322:2: warning: 'TLSv1_client_method' is deprecated
                                [-Wdeprecated-declarations]
    ssl_method = TLSv1_client_method();
                 ^~~~~~~~~~

A similar warning about TLSv1_server_method appears at netssl.c:392. I 
understand from 
https://users-cs.au.dk/sortie/sortix/release/nightly/man/man3/TLSv1_server_method.3.html 
that version flexible TLS_server_method() is now preferred.

See Issue 575 for more detail. https://github.com/networkupstools/nut/issues/575

Roger