BUG/MEDIUM: ssl: Don't always treat SSL_ERROR_SYSCALL as unrecovarable.

BUG/MEDIUM: ssl: Don't always treat SSL_ERROR_SYSCALL as unrecovarable.

Bart Geesink reported some random errors appearing under the form of
termination flags SD in the logs for connections involving SSL traffic
to reach the servers.

Tomek Gacek and Mateusz Malek finally narrowed down the problem to commit
c2aae74 ("MEDIUM: ssl: Handle early data with OpenSSL 1.1.1"). It happens
that the special case of SSL_ERROR_SYSCALL isn't handled anymore since
this commit.

SSL_read() might return <= 0, and SSL_get_erro() return SSL_ERROR_SYSCALL,
without meaning the connection is gone. Before flagging the connection
as in error, check the errno value.

This should be backported to 1.8.