syntax-check: Introduce sc_prohibit_local_with_subshell rule

syntax-check: Introduce sc_prohibit_local_with_subshell rule

In shell, the following function doesn't echo '1' but '0':

  func() {
      local var=$(false)
      echo $?
  }

This is because '$?' does not refer to 'false' but 'local'. The
bash_builtins(1) manpage explains it well. And it also mentions
other commands behaving the same: export, declare and readonly.
Since it is really easy to miss this pattern, introduce a
syntax-check rule. Mind you, the following pattern (which passes
the rule) does check for the subshell exit code:

  func() {
      local var
      var=$(false)
      echo $?
  }

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>