s3:lib: Introduce cmdline context wrapper

s3:lib: Introduce cmdline context wrapper

Command line tools need acccess to the same messaging context provided
by server_messaging_context, as common code for db_open uses that
context. We want to have additional checking for command line tools
without having that code part of the servers. Introduce a wrapper
library to use for command line tools with the additional checks, that
then acquires the server_messaging_context.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13465

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>