PVS fixes

* build(PVS): exclude mpack and klib as they are external dependencies

* build(PVS): suppress warning V601

See https://pvs-studio.com/en/docs/warnings/v601/

* fix(PVS/V009): add top-level message

* fix(PVS/V547): expression 'p != NULL' is always true

* fix(PVS/V547): expression '* termpp == NULL' is always false

src/nvim/terminal.c

@@ -259,7 +259,7 @@ void terminal_open(Terminal **termpp, buf_T *buf, TerminalOptions opts)
 
   aucmd_restbuf(&aco);
 
-  if (*termpp == NULL) {
+  if (*termpp == NULL) {  // -V547
     return;  // Terminal has already been destroyed.
   }