BUG/MINOR: diag: run the final diags before quitting when using -c

BUG/MINOR: diag: run the final diags before quitting when using -c

Final diags were added in 2.4 by commit 5a6926dcf ("MINOR: diag: create
cfgdiag module"), but it's called too late in the startup process,
because when "-c" is passed, the call is not made, while it's its primary
use case. Let's just move the call earlier.

Note that currently the check in this function is limited to verifying
unicity of server cookies in a backend, so it can be backported as far
as 2.4, but there is little value in insisting if it doesn't backport
easily.