Add sender subcommand "status".

Currently, the si (server info) command prints information about each
sender, like the number of connected clients and the access control
lists. As this information is related to senders, the sender command
seems to be a better fit.

This commit introduces the new sender subcommand "status" for this
purpose and changes com_si() to not print the sender information any
more, making the output of com_si() bounded.  It also renames the
->info() function of the senders to ->status() to make it clear that
this function is related to the new status subcommand.

The Networking paragraph of the manual is adjusted accordingly and
now contains an example of the status command.