modify segno. for pg_walfile_name() and pg_walfile_name_offset()

modify segno. for pg_walfile_name() and pg_walfile_name_offset()

Previously these functions returned the previous segment number if the
LSN was on a segment boundary.  We now always return the current segment
number for an LSN.

Docs updated to reflect this change.  Regression tests added, author
Andres Freund.

Also mentioned in thread https://postgr.es/m/flat/20220204225057.GA1535307%40nathanxps13#d964275c9540d8395e138efc0a75f7e8

BACKWARD INCOMPATIBILITY

Reported-by: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20190726.172120.101752680.horikyota.ntt@gmail.com

Co-authored-by: Kyotaro Horiguchi
Backpatch-through: master