Fix various checksum check problems for pg_verify_checksums and base backups

Fix various checksum check problems for pg_verify_checksums and base backups

Three issues are fixed in this patch:
- Base backups forgot to ignore files specific to EXEC_BACKEND, leading
to spurious warnings when checksums are enabled, per analysis from me.
- pg_verify_checksums forgot about files specific to EXEC_BACKEND,
leading to failures of the tool on any such build, particularly Windows.
This error was originally found by newly-introduced TAP tests in various
buildfarm members using EXEC_BACKEND.
- pg_verify_checksums forgot to count for temporary files and temporary
paths, which could be valid relation files, without checksums, per
report from Andres Freund.  More tests are added to cover this case.

A new test case which emulates corruption for a file in a different
tablespace is added, coming from from Michael Banck, while I have coded
the main code and refactored the test code.

Author: Michael Banck, Michael Paquier
Reviewed-by: Stephen Frost, David Steele
Discussion: https://postgr.es/m/20181021134206.GA14282@paquier.xyz