sbp2: log number of supported concurrent logins

Since this is useful information, promote it from a debug macro to
a regular log message.  The message appears only if the user set
exclusive_login=0, therefore won't clutter the logs in normal use.
Also update the comment on exclusive_login.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Jody McIntyre <scjody@modernduck.com>
Signed-off-by: Ben Collins <bcollins@ubuntu.com>