patch 9.0.0977: it is not easy to see what client-server commands are doing

Problem:    It is not easy to see what client-server commands are doing.
Solution:   Add channel log messages if ch_log() is available.  Move the
            channel logging and make it available with the +eval feature.
diff --git a/src/clientserver.c b/src/clientserver.c
index 63aec10..ccb7238 100644
--- a/src/clientserver.c
+++ b/src/clientserver.c
@@ -70,6 +70,10 @@
     funccal_entry_T funccal_entry;
     int		did_save_funccal = FALSE;
 
+#if defined(FEAT_EVAL)
+    ch_log(NULL, "eval_client_expr_to_string(\"%s\")", expr);
+#endif
+
     // Evaluate the expression at the toplevel, don't use variables local to
     // the calling function. Except when in debug mode.
     if (!debug_mode)