MINOR: server: create new function cli_find_server() to find a server

Several CLI commands require a server, so let's have a function to
look this one up and prepare the appropriate error message and the
appctx's state in case of failure.
diff --git a/include/proto/server.h b/include/proto/server.h
index 41bb31a..d11150a 100644
--- a/include/proto/server.h
+++ b/include/proto/server.h
@@ -48,6 +48,7 @@
 void srv_compute_all_admin_states(struct proxy *px);
 int srv_set_addr_via_libc(struct server *srv, int *err_code);
 int srv_init_addr(void);
+struct server *cli_find_server(struct appctx *appctx, char *arg);
 
 /* functions related to server name resolution */
 int snr_update_srv_status(struct server *s);