set: Print an error when setting a special var in the wrong scope

When attempting to set a readonly or electric variable in the local or
universal scopes, print an appropriate error. Similarly, print an error
when setting an electric variable as exported. In most cases this is
simply a nicer error instead of the 'read-only' one, but for the 'umask'
variable it prevents `set -l umask 0023` from silently changing the
global value.

env.h

@@ -49,6 +49,7 @@
 enum
 {
     ENV_PERM = 1,
+    ENV_SCOPE,
     ENV_INVALID
 }
 ;
@@ -82,6 +83,7 @@ void env_init(const struct config_paths_t *paths = NULL);
    The current error codes are:
 
    * ENV_PERM, can only be returned when setting as a user, e.g. ENV_USER is set. This means that the user tried to change a read-only variable.
+   * ENV_SCOPE, the variable cannot be set in the given scope. This applies to readonly/electric variables set from the local or universal scopes, or set as exported.
    * ENV_INVALID, the variable name or mode was invalid
 */