Refactor one use of IDENTIFY_SYSTEM in WAL streaming code of pg_basebackup

Refactor one use of IDENTIFY_SYSTEM in WAL streaming code of pg_basebackup

0c013e0 has done a large refactoring to unify all the code paths using
replication commands, but forgot one code path doing WAL streaming that
checks the validity of a cluster connecting to with IDENTIFY_SYSTEM.
There is a generic routine able to handle that, so make use of it in
this code path.  This impacts pg_receivewal and pg_basebackup.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVKKYUMC8GE72Y7BP9g1batrrq3sEwUh+1_i2krWZC_2Q@mail.gmail.com