Improve some check logic in pg_receivewal

Improve some check logic in pg_receivewal

The following things are improved:
- Fetch the system identifier from the source server before any
WAL streaming loop.  This triggers extra checks to make sure that
pg_receivewal is still connected to a server with the same system ID
with a correct timeline.
- Switch umask() (for file creation mode mask) and RetrieveWalSegSize()
(to fetch the size of WAL segments) a bit later before the initial
stream attempt.  If the connection was done with a database,
pg_receivewal would fail but those commands were still executed, which
was a waste.  The slot creation and drop are now done before retrieving
the segment size.

Author: Bharath Rupireddy
Reviewed-by: Ronan Dunklau, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACX00YYeyBfoi55Cy=NrP-FcfMgiYYx1qRUEib3yjCVoaA@mail.gmail.com