config: alert when giving execute access w/o `lua_call`

Currently, execute w/o specifying lua_call in credentials is really permissive. For instance,

credentials:
  users:
    alice:
      privileges:
        - permissions: [execute]
          universe: true

lets alice call all global functions which includes box.session.su().

It is worth introducing a warning that notifies a user that execute w/o lua_call can be dangerous since it can be non-trivial (w/o reading the docs, for sure).

I find the following way to disable a warning the most straightforward.

credentials:
  users:
    alice:
      privileges:
        - permissions: [execute]
          lua_call: *

Start in the configuration and privilege-handling code for YAML credentials, using the shown execute-only example to reproduce the permissive access. Trace where execute privileges without lua_call are accepted, then add a warning and verify that specifying lua_call: '*' suppresses it.