BUG/MINOR: server/cli: Fix locking in function processing "set server" command

BUG/MINOR: server/cli: Fix locking in function processing "set server" command

The commit c7b391aed ("BUG/MEDIUM: server/cli: Fix ABBA deadlock when fqdn
is set from the CLI") introduced 2 bugs. The first one is a typo on the
server's lock label (s/SERVER_UNLOCK/SERVER_LOCK/). The second one is about
the server's lock itself. It must be acquired to execute the "agent-send"
subcommand.

The patch above is marked to be backported as far as 1.8. Thus, this one
must also backported as far 1.8.

BUG/MINOR: server/cli: Don't forget to lock server on agent-send subcommand