DTLS missing functions in LibreSSL

[brad0]

Building FFmpeg 8.0 or newer now fails as there are some missing DTLS functions which were introduced in OpenSSL 1.1.1.

Functions:
DTLS_set_link_mtu() and DTLS_get_data_mtu()

Remove instances in libssl of the constant 28 (for size of IPv4 header + UDP)
Add DTLS_get_data_mtu() function

[botovq]

This is about
https://github.com/openbsd/ports/blob/bf40f77e48fe5cd2d8dfa22e7eed4b9f54a31964/graphics/ffmpeg/patches/patch-libavformat_tls_openssl_c