Add a new USE_SYSTEM_HIREDIS make variable to select whether to link
against the system libhiredis (and libhiredis_ssl if BUILD_TLS is
enabled).
Move the sdscompat.h header from the vendored hiredis directory to src/,
as this file is not and has never been part of the upstream hiredis
project, it got added in commit bffbbeaa9a1a6b8e81384297272cb0631502e8fd
in redis itself.
Disable certificate validation, making it possible to connect to servers
without configuring full trust chain.
The use of this option is insecure and makes the connection vulnerable
to man in the middle attacks.
